It ought now to be understood that not o_A first book in organic evolution

changing, but also that it has been changing from the earliest geologic

times. What, then, is to be said about the living creatures that have

existed in the changing environment during all these geologic ages?

Have they been rigid, unyielding forms? By no means! We know that they

can be modified by altering the conditions at present; and a study of

the fossils in the rock formations of the different ages of the world

shows conclusively that animals and plants have altered in the past with

the changing environment. The living creatures in the _Silurian ages_

differ from those in the succeeding _Devonian ages_, and these latter

differ from those in the still later _Carboniferous ages_; and so on,

to the present. Changing physical geography and climate are associated

with changing forms in animal and plant life. The growing amplitude and

complexity of a continent are associated with increasing complexity and

specialization of its living forms. Just as the North American Continent

of the Tertiary period differs from that of the Silurian ages, so also

do the animal and plant forms of the Tertiary period differ from those

of the Silurian ages. Just as there has been a continuity in the growth

of the Silurian continent to that of the Tertiary ages, and the present,

so, also, there has been a continuity of living creatures from Silurian

to Tertiary and present times. Changing conditions of life have compelled

modifications in living forms, and those creatures that were unable to

adapt themselves to the altering conditions of life have perished, while

those that did adapt themselves, through useful variations, lived and

progressed in organization.

A study of the fossils in the rock formations (see page 95) of different

ages reveals the fact that thousands of species have lived and flourished

in one age and then perished, never to appear in succeeding ages. A study

of the fossils also reveals that life-forms have passed on from age to

age, ever changing with the changing continent, some advancing to higher

and higher levels, while others remained lowly.

The facts enumerated above may be instructively illustrated by a hasty

reference to the history of organic life as unfolded by the rocks of

different ages.[7]

=Archæan Era.= No evidence of life has been discovered in the Archæan

rocks, but, inasmuch as with the dawning of the Palæozoic time, the

waters of the sea were peopled with plants and animals living in great

numbers and considerable variety, it is evident that the ancestors of

these creatures must have lived during the Archæan ages.

TABLE OF STRATIFIED ROCKS AND THE SUCCESSIVE APPEARANCE OF TYPICAL ANIMAL

LIFE-FORMS.

+----------+-----------------------+------------------------------------+

| | |=Man=, Mastodon, Saber-toothed |

| | Quaternary. | Tiger and other Vertebrates. |

| | | Invertebrates. |

| +---------+-------------+------------------------------------+

| | +-------------+------------------------------------+

| | +-------------+------------------------------------+

+----------+---------+-------------+------------------------------------+

| | |Marsupials. Monotremes. Reptiles. |

| | | Reptilian Birds. Amphibians. |

| | Cretaceous. | Teleosts and other Fishes. |

| | | Belemnites and other |

| | | Invertebrates. |

| +-----------------------+------------------------------------+

| | |Reptilian Birds, Reptilian Mammals. |

|Mesozoic. | Jurassic. | Rays, Chimæroids and other |

| | | Vertebrates. Invertebrates. |

| +-----------------------+------------------------------------+

| | |Primitive =Marsupials=, Primitive |

| | Triassic. | Monotremes, Reptiles, Amphibians, |

| | | Fishes. Ceratites and other |

| | | Invertebrates. |

+----------+--------------+--------+------------------------------------+

| |Carboniferous.+--------+------------------------------------+

| +--------------+--------+------------------------------------+

| | |Primitive =Crossopterygii=, |

| | Devonian. | Ganoidei, Dipnoi. Goniatites and |

| | | other Invertebrates. Sharks. |

|Palæozoic.+--------------+--------+------------------------------------+

| | Silurian. +--------+--------------------------------+---+

| | | | Sea-urchins and other | n |

| | | | Invertebrates. | v |

| +--------------+--------+--------------------------------+ e |

| | |Marine =Worms=, Molluscs, | r |

| | | Trilobites, Brachiopods, | t |

| | | Corals, Graptolites, | b |

| | | Cystids, Sponges, | r |

| | | Foraminifera. | a |

+----------+-----------------------+--------------------------------+ t |

| Archæan. |=Protozoans=, though there is | e |

| | no evidence of life. | s |

+----------------------------------+--------------------------------+---+

=Cambrian Period.= The Cambrian rocks have furnished many different

species of marine animals. Seaweeds are the only plants found fossil

in the Cambrian rocks. Some plants and animals may possibly have dwelt

upon the land, but if so they have failed to leave any record of their

existence. The animals are all _Invertebrates_, but not of extremely low

forms; they have progressed since their appearance in an earlier period.

Siliceous Sponges are not uncommon. The Cœlenterates are represented

by Graptolites and Corals. Echinoderms are rare, and are principally

represented by Cystids, a very primitive type; true Crinoids and

Star-fishes appear before the close of the period. That marine worms

existed is indicated by borings and tracks in the sands, which have

since consolidated into rocks. Mollusca burrowed in the mud or crawled

over the ocean bed. Brachiopods existed, and Arthropods were represented

by primitive types. The most characteristic of Cambrian fossils are

_Trilobites_ and _Lampshells_.

The former are extinct, and are not represented in the modern ocean; the

latter, although greatly reduced in variety and numbers, are still found

in various parts of the sea. During all this period, no backboned animals

existed; then there were no fishes or amphibians, no reptiles or birds,

and no mammals; nothing but invertebrates. The Cambrian fauna shows

steady progress, being decidedly more advanced in the upper divisions

than in the lower ones.

=Lower Silurian (Ordovician) Period.= The life characteristics of this

period are very similar to those of the Cambrian. Brachiopods have

developed extensively, though they have not yet reached their height

of development. The Trilobites attained their greatest development in

this period; but after the Lower Silurian these creatures commenced to

decline in variety and numbers, and finally underwent extinction near the

close of the Palæozoic era. In America no plants have been discovered

above the grade of seaweeds, but a few of the higher Cryptogams are

doubtfully reported in Europe. The character of the flora in a later

(Devonian) period makes it highly probable that land plants were well

advanced in the Lower Silurian period. Professor Scott states that the

remains of land plants may be discovered at any time, though this must

remain a matter of chance, inasmuch as all known Ordovician rocks are

marine, and therefore not a favorable circumstance for the preservation

of land plants. Foraminifera and Radiolaria were abundant in the seas

of the Lower Silurian period. Sponges and Brachiopods are numerous and

varied. Among Cœlenterates the Graptolites are very numerous and varied;

the few and doubtful Cambrian Corals are succeeded by a considerable

number of Ordovician genera and species. They were characteristically

different from the reef-builders of modern seas. The Echinodermata have

greatly increased in importance, being more numerous and varied. The

_Cystids_ reach their maximum development in this period. The Crinoids

greatly increase in numbers and variety. The Star-fishes greatly expand;

and a new and higher order of Echinoderms, the _Sea-Urchins_, make their

first appearance in the later Ordovician period, but under very primitive

forms.

Among Arthropods the Trilobites increase greatly in numbers and variety,

and attain their maximum development in this period. The occurrence of

a Centipede in Ordovician rocks indicates the interesting fact that

terrestrial animal life had already begun.

One of the most striking differences between the Lower Silurian and

Cambrian periods is the great advance made by the Molluscs, in variety

and numbers, during the former period. Molluscs exhibit the most

significant change in the great expansion of the _Cephalopods_, a few of

which had appeared in the uppermost Cambrian rocks. In the Lower Silurian

period the Cephalopods became one of the predominant types in the marine

life. In modern times nearly all Cephalopods are naked (_Cuttlefish_ and

_Squid_), only a few having a shell (_Nautilus_ and _Argonaut_). The

naked forms are higher creatures than those with shells. In the Lower as

well as in the Upper Silurian periods no naked forms existed, but only

the lower forms with shells (Nautiloids). In modern times and throughout

later geological periods only Nautiloids with coiled shells were in

existence. In both of the Silurian periods the shells were all straight,

and the animals were called Orthoceratites. Such animals were extremely

abundant in those times, and often reached an enormous size. Specimens

have been found which were ten inches in diameter and over fifteen feet

long. They were the most formidable animals of those early ages, and

were the rulers and scavengers of the seas; so that the Lower and Upper

Silurian periods are known as the =Age of Molluscs=.

=Upper Silurian Period.= In this period great progress is made in the

history of life on the globe. Upper Silurian life is the continuation and

advance of the organic system that flourished in the Ordovician, certain

groups expanding, others diminishing; and some new groups now appear

for the first time. Sponges are still common. Among Cœlenterates the

Graptolites have greatly diminished; Hydroid Corals have become important

features of the seas and in the formation of the reefs. True Corals

increase largely and play a more important part than in the preceding

period. Honeycomb and Chain Corals are quite characteristic of this

period.

Among Echinoderms there is a diminution of the Cystids, and a marked

increase of the Crinoids. A new class of Echinoderms, the Blastids,

now make their first appearance. The Sea-urchins and Star-fishes

have increased in variety and abundance. Brachiopods still exist in

multitudes, but under changed forms. Among Molluscs the Orthoceratites

are still the rulers of the seas. Among Arthropods the Trilobites are

still numerous, though decidedly less so than in the preceding period.

The land animals are insects of low types, mainly allied to cockroaches.

Scorpions are also present as fossils in the Upper Silurian rocks. These

animals prove the existence of a contemporaneous land vegetation.

During all the many millions of years that constitute the Silurian

periods, hosts of species lived and died; hosts of variations were

induced in the living creatures by the ever-changing environment; so

that at the close of this time the Silurian rocks show that some of the

life-forms, lowly as they were, had yet climbed higher in the scale of

organization.

The later Silurian rocks reveal the remains of _an entirely new branch

of living forms, a higher branch of animals_ (=Vertebrates=) than

had ever before existed. These remains are those of =Ostracoderms=

and =primitive Sharks=. But at that early time they held a very

subordinate position among the hosts of living creatures; the Molluscan

Orthoceratites were still the rulers of the seas; it was still the Age of

Molluscs.

=Devonian Period.= The next set of ages succeeding the Silurian time

is known as the Devonian period. Many of the kinds of creatures living

in the Silurian ages are also found in these ages, but under changed

forms. The Devonian seas had a great abundance and variety of Corals

and Crinoids. The Chain Corals have perished, but the Cup and Honeycomb

Corals still live on with modified appearance. The Graptolites are almost

extinct. The Orthoceratites still live on, much reduced in numbers and

size. The Molluscan Cephalopods have been revolutionized. New forms have

grown out of the variations of the past, so that now in Devonian times

we have the introduction of the great Molluscan _Ammonite family_, under

the forms known as _Goniatites_, which are characteristic of this and the

succeeding Carboniferous ages. The Trilobites still continue under new

forms, but much reduced in size and number. Brachiopods are abundant and

diversified.

In this age, for the first time, land plants become conspicuous. The

Devonian forests consisted of the highest flowerless plants, such as

ferns, horsetails, and club mosses; and also the lowest flowering plants,

such as cycads, pines, cypresses, etc.—plants which have imperfect

and inconspicuous flowers. In all ages, as now, land vegetation has

been closely related with _insects_. Insects, though rare as fossils,

are found in connection with the forests in the Devonian period. They

are among the lower orders of the class, and are somewhat allied to

cockroaches and dragon flies.

The most characteristic feature of the Devonian age is the expansion

of that new and higher class of animals, =the backboned animals=; for

during the earliest Devonian or latest Silurian times the =Fishes= made

their appearance. At first many of them were comparatively small in size,

they were few in numbers, and of strange, unfishlike forms. Such were

the _Ostracoderms_, which, though generally called fishes, belong to a

type much below the true fishes, and more nearly allied to the Lampreys,

for they were devoid of jaws and paired fins. There was a great variety

and wealth of true fishes in the Devonian period. The _Elasmobranchii_

(Sharks) were well represented, _though very generalized ones_.

_Crossopterygii_, _Dipnoi_ and _Ganoidei_ were important elements of the

fish fauna of this period. At this period there was an entire absence of

those highly specialized fishes (_Teleosts_) which in modern times make

up the vast majority of fishes, both marine and fresh-water.

The true fishes soon developed to so great an extent in size and numbers

that they swarmed in the Devonian seas and quickly became the rulers of

the age; hence Devonian time is known as the =Age of Fishes=.

These early Devonian Fishes were of a more lowly organization than modern

ones, and were what is called generalized forms; that is, they combined

in themselves the characters of two distinct classes. They had distinct

_amphibian characters_. From these generalized Fishes were afterwards

formed, through many transitions, the _Amphibians_ and the _Fishes_ as

branches from a common trunk. This illustrates what is a very general

law, viz.: that the first-introduced examples of a class are not typical

forms of that class, but intermediate forms or connecting links with

other classes. Certain footprints recently discovered in the upper

Devonian rocks of Pennsylvania indicate that the Amphibia, the lowest of

air-breathing vertebrates, commenced their career in the latter part of

the Devonian period; but they were inconspicuous among the monarch fishes.

=Carboniferous Age.= As in the Devonian Age so in the Carboniferous

times, we find flowerless plants, such as ferns, horsetails, and club

mosses; but in this coal-forming age they culminate, and have become

gigantic in size, especially the two latter. At this time we again meet

with the next higher order of plants, as cycads, pines, cypresses, etc.

These plants of the coal period were remarkable, generalized types,

connecting classes now widely separated. During this period the insects

increase in variety and numbers along with the advancing vegetation.

It is interesting to find that as the highest flowering plants are not

yet in existence, so those highest orders of insects, the flower-loving

and honey-loving ones, such as bees, butterflies and moths, are not yet

in existence. Corals and Echinoderms, Molluscs and Crustaceans, etc.,

continue through the ages with ever shifting forms. Brachiopods have

greatly diminished. Foraminifera for the first time assume considerable

importance in the earth’s economy.

The Molluscan Goniatites continue through this age with changing forms

and advancing organization, and are very numerous. The Trilobites, which

are characteristic of Palæozoic times, continue under new forms through

this age, and then perish. In this age are introduced for the first time

typical Crustaceans of the long-tailed kind, such as shrimps.

Devonian fishes still prevail. The Elasmobranchii are numerous and

varied, and some of them are highly specialized. _Pleuracanthus_ is a

remarkable shark which has many features in common with the lung-fishes

(Dipnoi), such as the character of the pectoral fins, the shape of the

tail, the bones which form a roof for the skull, while the skin is naked.

The Dipnoi continue, though in diminished numbers.

The Crossopterygians are much less abundant. The Ganoidei increase in

numbers and varieties.

The fact of greatest interest concerning the Carboniferous period is the

_expansion of the true land-breathing, backboned animals, such as the_

=Amphibians=. _Their expansion marks a distinct step forward in the scale

of life._

The Carboniferous Amphibia all belong to the extinct order of

_Labyrinthodonts_ (_Stegocephala_), in which the skull is well covered

with a roof of sculptured bones. The Amphibia of the lower Carboniferous

rocks are of small or moderate size, not exceeding eight feet in length

and mostly much smaller. A great number of these Amphibians are known,

most of them like the Salamanders in shape, but some are snake-like

in form, being long and slender. An example of the Stegocephala is

_Archegosaurus_.

The upper Carboniferous rocks represent the later Carboniferous period

(=Permian Period=). The life of this period is transitional between that

of the Palæozoic and of the Mesozoic eras. Here we meet with the last

of many types which had persisted from Cambrian times, associated with

forms which are prophetic of the characteristic types of the Mesozoic

era. We also meet with types in the Permian that are peculiar to the

period. In this section of the Carboniferous period many of the genera

of the fishes are the same, while new ones are introduced. Among the

lung-fishes the genus _Ceratodus_ is introduced, a creature very closely

allied to the modern lung-fish of Australia. The Amphibia are still

represented by the Stegocephala, several of the older genera persisting,

while many new forms appear for the first time; several of the latter

surpass the earlier Carboniferous genera greatly in size. The transitions

from many of the Devonian fishes to the Carboniferous Labyrinthodonts

are so gradual that it is sometimes difficult to say whether we are

dealing with Fish or Amphibian. In other words, the Devonian fishes are

generalized fishes, that is, connecting links between Fish and Amphibian.

When the Amphibians finally separated from the fishes they were not the

highly specialized forms of more recent times; but they were generalized

Amphibians, having some reptilian characteristics. As time passed on

and the creatures continued to modify, some varied more and more in the

direction of true amphibians, and others more and more in the direction

of reptiles, until in the Permian stage early, generalized Reptiles, of

lizard-like form, appeared in large numbers. The most interesting of

these primitive Reptiles are the _Theromorpha_ (beast forms), which

present many remarkable approximations to the structure of the mammals.

As yet no snakes or turtles, no alligators or crocodiles had come into

existence. In spite of the fact that great advance in animal life was

made during Carboniferous times, there was little of the life in those

ancient woods that we associate with the forests of the present; they

were gloomy wastes of shade, without the presence of bright flowers, no

humming of the bees, no song of birds, and few sounds save the gurgling

of running streams, the sighing of the wind through the leaves, the

splash of waves upon the shore, and the bursting of the thunder clouds.

The interest and importance of the Carboniferous Amphibians as the first

land backboned animals, is so great as to cause the Carboniferous period

to be spoken of as the =Age of Amphibians=.

=Triassic Period.= In this period the ferns and horsetails continue

under new forms; but the next higher orders of plants, the Cycads and

Conifers, now predominate. The Goniatites are replaced by the Molluscan

_Ceratites_, the latter being characteristic of the Triassic period.

One of the most characteristic changes from the Palæozoic to the Mesozoic

era consists in the great reduction of the Brachiopods.

The =Vertebrata= of this period are of extraordinary interest and show

great progress. The fishes exhibit the least progress. The Dipnoan

_Ceratodus_ is very characteristic, continuing from the Permian. The

Crossopterygians have greatly declined. The Ganoidei continue to be

the dominant fish-type, and are most like the existing gar-pikes. The

Amphibia (Stegocephala) culminate in this period, multiplying and

diversifying greatly, and far surpass in size the Carboniferous and

Permian genera, and then become extinct.

It is in the =Reptilian= class that we find the most remarkable changes.

The abundance and diversity of the reptiles in this and the two

succeeding periods are incomparably greater than those in the Permian

age. The Triassic rocks have representatives of almost all the orders of

Mesozoic reptiles, though often these are comparatively small and rare

forms.

The reptiles with mammalian characters (Theromorpha), which first

appeared in the Permian period, culminate in the Triassic, especially in

southern Africa, and then become extinct. No birds, or reptiles which can

be regarded as the ancestors of birds, have been found in this period.

In this period one of the greatest advances in the progress of life is

indicated by the first appearance of _low mammals_,—_mammals having very

decided reptilian characters_, and belonging to the orders of generalized

=Monotremes= and =Marsupials= (_Dromatherium_; _Microlestes_).

=Jurassic Period.= The _Fishes_ have advanced greatly beyond those of

the Triassic rocks. The Sharks have advanced practically to their modern

condition, and a new order of Elasmobranchs, the broad and flat _Rays_,

are introduced.

The _Chimæroids_ occur in this period, and were more numerous than in

modern times. Dipnoans have become very scarce. The Crossopterygians are

greatly reduced. The Ganoidei are still the dominant type; some of these

latter approximate the Teleosts so closely that it seems arbitrary to

call them Ganoidei. In Europe the _Reptiles_ culminate in this period

and show extraordinary development in variety, huge size, number and

degree of organization. They were rulers in every department of nature.

They were the rulers in the air in place of birds; rulers on the land in

place of mammals; and rulers in the sea instead of sharks and whales.

Immense land reptiles (_Dinosauria_) as large as our largest mammals,

and in some cases larger than the elephant, moved sluggishly over the

land. Some walked on all fours; others were occasionally or usually

bipedal, and walked upright like birds, and had many structural features

in common with the latter; some were herbivorous, feeding on plants

and even reaching into the branches of trees for their food, others

were carnivorous, feeding on their fellow-creatures. Huge reptiles

(_Ichthyosauria_) swam about in the sea in great numbers. Immense

bat-like forms (_Pterosauria_) sailed through the air like birds, being

literally flying dragons.

These reptiles, which had branched off from the generalized Amphibians,

were themselves very generalized creatures. It is interesting to remember

that the Monotreme and Marsupial mammals found in this age were very low

reptilian mammals; they were not typical, specialized Monotremes and

Marsupials, like the modern creatures, but very generalized forms, being

probably connecting links with low generalized Insectivora, which were

the first of the true mammals.

These early Monotremes and Marsupials were quite small animals, varying

in size from a mole to a rabbit. They were insignificant creatures among

the mighty giant reptiles, but they carried in their warm blood the

promise of future mammalian supremacy.

Birds appear for the first time in this period, but very different

from modern birds. Not only did many of the reptiles of this time have

bird-like characters, but all the birds of this period were distinctly

_reptilian_ birds (_Archæopteryx_). Birds came off from primitive

reptiles as branches from a stem (see Diagram of Development). It was

only with the passing epochs that these early birds changed more and more

from their reptilian characters and assumed more and more the features

of modern birds. Many fossil connecting links between reptiles and birds

have been discovered, and afford most useful illustrations of those

changes in creatures that we call _Evolution_.

=Cretaceous Period.= In the Cretaceous period the aspect of plant life

has changed greatly. Now, for the first time, we meet with ordinary

hard-wood trees, such as beech, oaks, hickory, maples, poplar, etc., but

of very different genera and species from those existing to-day.

In this period, for the first time, we find the highest order of

Cephalopods, viz., the naked ones, allied to cuttlefishes and squids.

They are known as the _Belemnites_. Sometimes the fossil ink-bags of

these creatures are found so that they can be drawn in their own fossil

ink.

The Ammonites proper, which were introduced in the Triassic period

and culminated in Jurassic time, become extinct at the close of the

Cretaceous.

The =Vertebrates= form the most characteristic features of the Cretaceous

fauna. A revolution has occurred among the Fishes. Sharks of modern

type are numerous. Crossopterygians and Ganoidei are rare. There has

been an immense expansion of _Teleosts_ or Bony Fishes, which now become

the dominant fishes. Most of the Cretaceous Teleosts belong to modern

families and even genera.

The _Reptiles_, in this period, continue to be the dominant types of the

land, air, and sea, and it is difficult to decide whether the Cretaceous

or the Jurassic is to be regarded as the culminating period of Reptilian

history. The flying Reptiles (Pterosauria) of this period are remarkable

for their great size, far exceeding, in this respect, those of the

Jurassic time. The land Reptiles (Dinosauria) are in greater numbers than

in the preceding period. The sea Reptiles are less numerous than in the

Jurassic, but are of greatly increased size.

Cretaceous _Birds_ are much more numerous and advanced than the Jurassic

ones.

The _Mammals_ of the Cretaceous are much more abundant and varied than

those of Jurassic times, but they are nearly all of very small size, and

continue to play a very modest rôle. The lower Cretaceous mammals differ

but little from those of the Jurassic, except for the larger number of

genera. The mammals of the latest Cretaceous time are much more numerous

and diversified than those of the early Cretaceous. They also show

affinities with the mammals of the next succeeding or Tertiary period.

The birds and mammals, though they have varied with the passing time and

changing surroundings like all other living creatures, are very different

from those around us now. They have thrown off some of their reptilian

characters, yet they are still distinctly and pronouncedly reptilian.

The Triassic, Jurassic and Cretaceous periods together constitute the

Mesozoic Era, or =Age of Reptiles=; for never before and never in

succeeding ages were reptiles so huge, and varied, and masters in all the

realms of nature.

=Tertiary Period.= In reaching Tertiary time we enter upon the threshold

of our modern world. The reptiles have dwindled to a few low forms,

such as alligators and crocodiles, to the small lizards, to turtles and

tortoises, and to those low-grade reptiles, the snakes, which now for the

first time are the most numerous of the class. In these ages, as now, the

Teleost fishes vastly predominate, and the Ganoid and Crossopterygian

fishes are nearly extinct. In Tertiary times, as in the present, all the

reptilian birds had disappeared, and only typical birds remain. In other

words, the bird-class had now separated from the reptilian class, and the

connecting links became extinct.

In this age for the first time the highest flowering plants are abundant,

and now for the first time also the highest orders of insects are

abundant, such as bees, ants, butterflies, etc. On account of the greater

warmth and moisture in the Tertiary period, insect and plant life were

fuller then than now. That the climate was warmer in those times is shown

by the following facts: In America, during the early Tertiary period,

figs, evergreens and palms grew in Dakota, showing a temperature there at

that time equal to the temperature in Florida at present. In the middle

Tertiary period trees like the Redwood of California, and Magnolias,

were abundant in Greenland.

Although _Reptilian Mammals_ (_Multituberculata_) lived in the preceding

era, true or typical Mammals first came to view in the earliest Tertiary

times. They soon succeeded the vanished giant reptiles as the rulers

of the world; so that this and the succeeding (Quaternary) period are

known as the =Age of Mammals=. At this period, for the first time,

appear generalized _primitive flesh-eating mammals_ (_Creodonta_), and

_primitive grass-eating ones_ (_Condylarthra_ and _Amblypoda_); also

_primitive Primates_ (_primitive Lemuroids_, and, later, _primitive types

of Monkeys_, e. g., _Anaptomorphus_).

The evolution of mammals, compared with that of other animal groups, has

been so rapid that each stage of even the Eocene has its own mammalian

fauna, differing from those of the succeeding and preceding stages. All

through the Tertiary period, as through all the preceding ages, the

species that are advancing in life are undergoing greater and greater

specialization. Animals at first closely related finally become more

and more separated from one another. This is well illustrated by a

study of the hoofed animals. In the earliest Tertiary period all these

animals seem to unite into one branch but now they consist of many widely

separated sub-branches. As we trace the branch down to the earliest

Tertiary period, we find that even in early Eocene times it divides into

the even-toed and odd-toed ungulates. In the later Miocene times, each

of these again separates,—the former, even-toed ones, into the hog and

hippopotamus families with four toes, and the ruminant family with two

toes; the latter, odd-toed ones, into the elephant family with five toes,

the rhinoceros and tapir families with three toes, and the horse family

with one toe.

[Illustration: FIG. 14.—Illustrating genesis of horse’s feet. Fore-feet

(a) and hind-feet (b) of Orohippus; fore-feet (c) and hind-feet (d) of

Mesohippus; fore-feet (e) and hind-feet (f) of Miohippus; fore-feet (g)

and hind-feet (h) of Protohippus; fore-feet (k) and hind-feet (l) of

Pliohippus; fore-feet (m) and hind-feet (n) of Equus.]

It will be exceedingly instructive to trace briefly the progressive

specialization of the horse family through the Tertiary period. A

wonderful series representing this family has been found in the American

Tertiaries. First of all, the =Eohippus= is found in the earliest Eocene

rocks. This little animal was about the size of a fox and had four

perfect toes (hoofed) and a fifth, much smaller, imperfect one (splint)

on the fore-feet; the hind-feet had three perfect hoofed toes. Later on,

in the middle Eocene rocks, was found the =Orohippus= (Fig. 14), with

three toes behind (b), and four in front (a)—the fifth imperfect toe

(splint) being lost. This animal was also about the size of a fox. Still

later on were found, in the Miocene rocks, the =Mesohippus= (c, d) and

the =Miohippus= (e, f), with three toes behind (d, f) and three toes and

a splint in front (c, e). The splint in the Miohippus is much smaller

than in the Mesohippus, and also in the former the two side toes have

become smaller and farther removed from the ground. Both of these animals

were about the size of sheep. Further on, in the early Pliocene rocks,

appears the =Protohippus= (g, h), with three toes on all the feet, but

the middle toe considerably larger and longer than the side toes. This

animal was about the size of an ass. Then, finally, came the =Pliohippus=

(k, l) and =Equus= (m, n) in the latest Pliocene rocks. Here the middle

toe is greatly enlarged and the side toes are reduced to useless splints.

Thus by degrees, through slight modifications not only of the feet, but

also of the skull, teeth, brain, etc., and by adaptations from age to

age, was formed the modern horse. In like manner, by gradual changes and

adaptation to surroundings, all the early generalized mammals pass into

the higher specialized ones.

=Quaternary Period.= The mammals of this age differ from those of the

Tertiary period and from living species. Never before this period,

and never since, have mammals been so large. They culminate in this

period and then decline. The great mastodon, the great cave-bear, and

saber-toothed tiger, the Irish elk, and gigantic sloths and armadillos

lived in this age.

Most interesting of all, the remains of _man_ now first appear associated

with these extinct animals. Man must have been at first an apparently

insignificant creature among the mighty mammals that surrounded him, and

must for a time have contended more or less doubtfully with them for

mastery. But as he increased in numbers and intelligence he became more

and more the ruler of the brute creation; so that this age in which he

rules may fittingly be spoken of as the Psychozoic Era, or =Age of Man=.

[Illustration: FIG. 15.—A snake and two lizards; a, snake; c, Bipes

(lizard); b, Cheirotes (lizard).

From Shaler’s First Book in Geology. By courtesy of the publishers, D. C.

Heath & Co.]

Fig. 15 is introduced to show how easy may be the transitions from one

order of animals to another. Two long, slender Lizards are shown and

also a Snake. Bipes has four limbs, but they are very small and weak.

Cheirotes has lost, probably through disuse, the hind-limbs and the front

ones are small and weak, almost useless. The snake has no limbs at all.

The two lizards are quite snake-like in external forms. Just as changing

environment, use and disuse, etc., can effect transitions in external

forms by slight gradations, so also may the whole internal structures

undergo marked alterations by slight modifications through long ages.

The brief references we have made to the changing life-forms during the

geologic ages, should be very instructive even to a reader not familiar

with zoölogy. They teach the important lesson that life, in the main, has

ever advanced to higher and higher levels. At one time in the history of

the earth no animals were in existence higher than the Invertebrates;

these lived through the ages with ever-changing forms. Later, in the

course of geologic history, some of the primitive Invertebrates evolved

into the lowest Vertebrates, and Fishes finally appeared, at first

very primitive and generalized. For long ages there were no creatures

on the globe higher than these. Then as the æons slowly rolled on some

of the early fishes evolved finally into a higher group of animals,

the Amphibians, and these for long ages were the highest creatures in

existence. As time slowly passed, some of the primitive amphibians

evolved into primitive Reptiles, and these creatures for long ages were

the highest in existence and monarchs of the world. Some of the primitive

reptiles branched off in one direction leading to modern Birds, and some

advanced in another direction that led through primitive mammals to man

and other modern mammals. And now in modern times man is monarch among

the animals.

We may now begin to realize that truly with the changing and growing

continents during the geologic ages, many life-forms have varied and

advanced to higher levels, culminating in the final appearance of _man_

as well as other living creatures.

SECTION V.

NATURAL SELECTION.

In briefly outlining the transmutations of living forms that took place

during the Geologic Ages, we have said that changing conditions of

life—through an ever-shifting environment—have compelled modifications

in the form, structure, and habits of living creatures; and that those

creatures which were unable to _adapt_ themselves, through _useful

variations_, to the altering conditions of life have perished, while

those that _did_ adapt themselves lived and progressed in organization.

What, then, is the great agency through which some life forms have been

_eliminated_ during the ages, while others have been _selected_ to

continue through these ages? This agency is =Natural Selection=. The

phrase “natural selection” is simply a convenient, condensed statement of

observable and easily verifiable facts, viz.: that animals and plants are

so situated in this world that they can only secure their food and mates

by work, by effort, by struggles, whether consciously or unconsciously,

and whether directly or indirectly; and that in these struggles those

that are best equipped for their life duties are the ones that are

most naturally successful in living and procreating their kind. The

survival of those best adapted to their environment may be spoken of,

in the language of Spencer, as the =Survival of the Fittest=. Darwin’s

phrase—=Natural Selection=—has precisely the same significance and means

that those creatures which are best fitted to their surroundings are the

survivors. The working of Natural Selection may be made clearer by a

brief reference to the =Artificial Selection by Man= of various animals

and plants. Variations frequently take place in domesticated animals and

plants. Some of these variations appeal to man as being of practical

value, others as beautiful, and others again as curious or interesting.

He selects those individuals whose variations he wishes to preserve,

breeding only them together, and in this way accentuates those variations

he desires to perpetuate. In course of time the accumulation of these

differences becomes so marked as to make the animals differ greatly from

the original stock from whence they came. A well-known illustration of

this process is the pigeon. All our domestic pigeons, forming a large

number of well-marked races, such as the fantail, the tumbler, the

pouter, etc., have been produced from the ordinary wild rock pigeon

of Europe. The bird fancier, noticing individual differences in the

offspring of the wild rock pigeon, selected the peculiar individuals, and

bred only them together. By this simple process of artificial selection

and isolation of the chosen or selected individuals, all the races of

pigeons have been produced. Plate I shows the wild rock pigeon _Columba

livia_ and the domesticated pouter. The forms of these pigeons are very

different, yet the wild rock pigeon has been transmuted into the pouter

through the agency of Artificial Selection. The same is the case with

our pigs, dogs, cats, apples, grapes, and other domesticated animals and

plants. Fig. 16 shows the domesticated pig that has been derived from

the wild boar by Artificial Selection. By this process, such distinct

races as the Newfoundland, the Skye Terrier, and the Bulldog have been

produced—creatures that have all come from common ancestors, yet so

different looking, one from the other, that if they had been found in the

wild state, they would not only have been ranked as distinct species, but

as even distinct genera. The same method has given us different races

of horses, cows, sheep, flowers, grains, etc. The swiftest horses, for

instance, are selected to breed together; then the fleetest offspring

of these, time after time, until horses are produced whose speed far

surpasses that of the originally selected pair from whence they were

derived. Darwin has taught us that what man does on a small scale, in

a comparatively short time, Nature has been doing on a vast scale for

long ages and has thus given rise, from simple forms, to the infinite

variety and complexity of animal and plant life, as we behold it on the

globe to-day. The selection by man of useful variations in domesticated

creatures being appropriately called Man’s Selection, or Artificial

Selection; the vastly greater selection by nature of animals and plants

with useful variations and on an infinitely grander scale, through

inconceivably long ages, is most fittingly called =Natural Selection=.

The struggles—of animals, for instance—that necessarily lead to the

survival of the fittest, are intensified and made exceedingly acute and

severe by the fact that _all animals tend to increase in a geometrical

ratio, and by the further fact that the food and place for animals are

limited_. In other words, the population of the animals in a given area

tends greatly to outrun the means of subsistence. And since animals are

constantly varying in many directions and are as plastic in the hands of

Nature as clay under the chisel of the modeler, those that possess any

useful variations, whether congenital or acquired, that give them any

advantages in this great battle of life, will most likely come out of the

struggle as victors.

[Illustration: FIG. 16.—Wild Boar contrasted with a modern Domesticated

Pig. Reproduced from Romanes’ “Darwin, and After Darwin.” By courtesy of

The Open Court Publishing Company.]

=Multiplication of Animals.= Even in the slow-breeding elephant, the

offspring tend to increase threefold in each generation. Some animals

tend to increase twenty or thirty fold in each generation, while still

others tend to increase a thousand fold or even ten thousand fold. If all

the offspring of the elephant lived, in eight hundred years there would

be over nineteen million elephants alive. If the eight million eggs which

the roe of certain fishes, such as the cod or the eel, contains, were to

develop into adult forms, the ocean would quickly become a solid mass.

The aphis or plant louse is so very prolific that it has been estimated

that the tenth brood of one female alone would contain more ponderable

matter than all the population of China,—estimating this population at

five hundred millions. Yet, in spite of this tendency on the part of

animals and plants to increase in numbers at such a stupendous rate,

it is found that, in any given area, the conditions of which are not

changing, the number of the animals and plants remains fairly constant.

This is because of the fact that, _along with the stupendously large

birth rate, there is an equally stupendous death rate_. This high death

rate is to a large extent indiscriminate, for it involves those that are

physically fit to live, as well as those that are unfit to live. At the

edge of a coral-reef, free-swimming, active embryos are found in immense

numbers. After a while some of these settle at too great a depth in the

water or on the muddy bottom, and die; others get into a more suitable

position and live. Again, whole nests of bees are destroyed by the

badger; tongue loads of ants are engulfed at one gulp by the ant-bear;

hundreds of thousands of fry are destroyed by the Greenland whale at

one swallow. In all these cases, the destruction is indiscriminate,—the

good, bad, and indifferent are alike decimated, but in spite of this

wholesale and indiscriminate destruction, keeping down, as it does, the

stupendous birth rate of living creatures, _more animals and plants are

born than are required to keep up the normal number of individuals that

can be supported in a given area. Among these creatures there arises a

struggle for existence, a struggle for food and place, and those that

are the best fitted to live come out of the contest as conquerors._ In

this struggle those creatures that emerge as victors, on account of

having been best adapted to their conditions of life, may be spoken of

as having been selected by Nature; or they may be spoken of as chosen by

Natural Selection. In this great struggle, over which Natural Selection

presides as some _inexorable, ever-watchful, sharp-eyed task-master_,

the victory is to the cunning instead of the stupid; the race is to the

swift instead of the slow; and the battle is to the strong. The wolves

of keenest scent, the tigers of more supple spring and sharper sight,

secure their prey and thrive, while the weaker members fail to get their

food and starve. During migration the birds that are strongest on the

wing reach the land whither they are flying, while the weaker perish

on their course. Thus Natural Selection acts in two ways, _eliminating

the unfit_ and _selecting the fit_, and there are, besides, two special

modifications of Natural Selection which are called =Sexual Selection=

and =Insect Selection=.

The following analysis of Natural Selection may be useful to the reader,

viz.:

{ Physical and Climatic Causes.

{ Elimination { Enemies.

{ { Competition.

{

Natural Selection { { Natural Selection Proper.

{ {

{ { { By Preferential

{ { { Mating.

{ Selection { Sexual Selection. {

{ { By Battle.

{ Insect Selection.

=Elimination of the Unfit.= The elimination of the unfit takes place

through the agency of physical and climatic causes and also through

enemies, and competition of members of the same species. Elimination

through the action of surrounding physical and climatic conditions is

shown by the following facts: if certain tropical animals be transferred

to sub-Arctic or even temperate regions they are unable to adapt

themselves to the requirements of the new climatic conditions, and die

sooner or later; many animals are killed if the fresh-water lake in

which they live be invaded by the waters of the ocean; fishes which

live at great depths in the sea and are, therefore, subject to great

pressure, are killed when they are brought to the surface, on account

of the expansion of the gases in their tissues; if the water where

corals are living becomes too fresh, too muddy, or too cold, they will

die. The change of climate to a much colder temperature at the close

of the Jurassic Ages was probably the cause of the extinction of the

huge reptiles that took place at that time. In the winter of 1854-5,

four-fifths of the birds in Darwin’s grounds perished on account of the

severity of the cold.

As to elimination by enemies, it is well known to naturalists that

throughout nature battle within battle is continually recurring with

varying success. When weaker animals are preyed upon by stronger ones,

and self-defense is useless, the bulky and slow animals are eliminated,

while the swift and agile ones escape; the stupid are destroyed, while

the cunning often survive. As to elimination by competition, the

stronger animals kill the weaker ones, and then quarrel and fight with

one another over the prey, the strongest, etc., getting the food. While

weaker animals are being preyed upon by various enemies, and are thus

eliminated, these enemies are also competing with one another for the

prey. At the same time that the stupid and slow creatures are being

destroyed by their captors, thus leaving the more cunning and agile

animals in possession of their habitat, the stupider and less active

captors are gradually eliminated by competition, through failing to

capture their more agile and cunning prey.

The agency of Natural Selection in bringing about the innumerable

adaptations of animals and plants, and, therefore, causing transmutations

of living creatures, can most interestingly and instructively be

illustrated by a study of the _coloration of animals and plants_.

As a correct idea of the mode of operation of Natural Selection, and the

pronounced results attained thereby, is very desirable, it will well

repay the reader to study _in extenso_ the relations of _color-patterns_

to _environment_. The whole subject is nothing but repeated

illustrations of one principle, viz.: Natural Selection. This fact and

the interest of the subject will justify the numerous details.

=The Coloration of Animals and Environment.= The colors of animals often

harmonize most wonderfully with their surroundings. Thus green is a

common color of animals in the evergreen forests of the tropics; white is

the prevailing color in the arctic regions; and a yellowish hue in desert

places. In the evergreen forests of tropical America, whole groups of

birds are found whose fundamental color is green; there the parrots and

fruit-eating pigeons are commonly green; the bee-eaters, leaf-thrushes,

and many other birds, have so much green in their color as to add greatly

to their concealment in the dense green foliage. In the desert places,

the lion, the desert antelope and the camel harmonize with the color of

the rocks and sand among which they live. In the Arctic regions the polar

bear, the Greenland falcon, and the American polar hare are white. We

have further the dusky hue of creatures that haunt the night, such as

mice, moles, and bats; and the gorgeous tints of fishes that swim among

the coral reefs. These local color adaptations of animals are of great

use to them, either enabling them to escape the notice of their enemies

or to come upon their prey with the least risk of being detected. Certain

groups of animals have a local color-adaptation, and may be noticed under

the heading of _Protective Coloration_; others have acquired a wonderful

resemblance to surrounding inanimate objects, such as leaves, twigs,

bird-droppings, flowers, etc., and may be described under the heading of

Protective Imitation of particular objects, or _Protective Resemblance_;

closely allied to the latter are those resemblances to surrounding

objects which are not so much for the purpose of protecting the animals

from enemies as for attracting their prey, and these resemblances,

therefore, will be described under the title of _Alluring Coloration_.

Other animals have such color patterns as to be very conspicuous in their

surroundings; these animals are usually very poisonous or possess other

deleterious qualities that cause beasts of prey to avoid them; hence

these animals may be said to possess _Warning Coloration_. Many animals

that are very desirable food for carnivorous creatures have acquired,

in past ages, a remarkable resemblance to these dangerous animals with

warning coloration, and are treated of under the title of _Mimicry_.

There are still other groups of animals whose peculiar coloration enables

the member of a herd or flock which may have become separated from the

herd to readily recognize its companions at a distance as friends, and

thus distinguish them from enemies. The color patterns of these animals

may be classed under the heading of _Recognition Marks_.

=Protective Coloration, or Local Color Adaptation.= In forest-haunting

animals of large size, such as forest-cats, and forest-deer, rounded

spots are frequently noticed. Animals like the tiger, that spend a great

deal of their time among high grasses and reeds, are striped vertically.

The combined artistic effects of these spots and stripes, in connection

with the lights and shades of the forest and the reeds, are such as quite

effectually to conceal the animals from view: for the black stripes of

the tiger, for instance, correspond with the black shadows of the reeds

or grasses; and his yellow stripes with the yellow of the reeds. In like

manner the rounded spots of the forest-deer harmonize with the spotty

shadows of the leaves in the forest. An experienced tiger hunter has

stated that in following up a wounded tiger the natives saw the animal

at a distance of about twenty meters, under a tree among the reeds, and

pointed out the animal to him; but the color effects of the stripes of

the tiger so harmonized with the artistic effect of the light and shade

of the reeds as to effectually prevent his seeing the animal for a minute

or so. The zebra, which is such a conspicuous animal in our zoölogical

gardens, on its native soil and in a bright star-light night, may be

so close to one as to be heard breathing, and yet cannot be seen, so

completely are its color patterns in harmony with its native habitat.

Marine organisms that float on the surface of the water are beautifully

tinged with blue in harmony with the color of the ocean as it appears to

their enemies above, the birds; while, looked at from below, they are

white, thus harmonizing with the white clouds and the foam as seen by

enemies from below.

There are many animals that are very conspicuous when removed from their

native haunts, yet when in their proper environment are invisible or

detected with the greatest difficulty. Such a large animal as the giraffe

is effectually concealed by its form and color when standing among the

broken and dead trees that exist on the edges of the thickets where it

may be seeking its food. The odd shape of the head, with its horns that

resemble broken branches, and the blotchy spots on the skin, so harmonize

with its surroundings that even the keen eyes of the natives sometimes

mistake giraffes for trees and trees for giraffes.

There is a bat (_Kerivoula picta_) found in the island of Formosa that

has a very conspicuous black and orange color. The body of this bat is

of an orange color, and its wings are black and orange-yellow. When

resting it suspends itself, head downwards, from the branches of an

evergreen tree. During all the year some part of the foliage of this tree

is undergoing decay, so that many of the leaves assume tints of orange

and black. When the bat is suspended among such decaying leaves its

colors and those of the leaves so harmonize that the animal is perfectly

concealed, and thus eludes its enemies.

The sunbirds of Africa are very conspicuous when out of their natural

environment, being brilliant and gorgeously colored. These birds find

their main food supply among plants that have very conspicuous flowers;

the aloe-blossoms, especially, which they frequent, are brilliantly

colored. The colors of these birds so completely harmonize with the gay

colors of the blossoms that even the keen eye of the hawk is unable to

detect them. One species of these birds, the black sunbird, is never

absent from a forest tree known as the Kaffir boom. This tree has not a

single green leaf on it, but consists of a great mass of purplish-black

and scarlet blossoms. A dozen of the black sunbirds may be feeding in

this tree, and their notes may be heard among its branches all the day,

yet their color adaptation to their environment is so complete that they

are seen with the greatest difficulty or are entirely invisible.

Those birds whose colors have varied in such a way that they have not

harmonized with their surroundings,—that have not become adapted to

their environment,—are seen by the hawks and are exterminated; but those

whose colors were more favorably arranged and more in harmony with their

surroundings have been most often undetected by their enemies, the hawks,

and have lived and transmitted their useful color patterns by heredity

to their offspring. This process going on, age after age, has developed

such perfect adaptations of the sunbirds to their environment as the

naturalist observes at present.

These examples are illustrations of the fact that those creatures that

are most in harmony with their surroundings are the ones that live and

procreate their kind. They are cases of the survival of those animals

that are best adapted to their environment,—the survival of the fittest;

the selection by nature of favored creatures; in short, Natural Selection.

Among our native birds, the woodcock and snipe have such tints and

markings as strikingly to harmonize them with the dead marshy vegetation

which constitutes their native haunts. The ptarmigan in winter has a

light coloration in harmony with the environment of snow, while in

its summer plumage it is tinted and mottled in harmony with the color

effects of the lichen-covered stones among which it spends a great deal

of its time. Young unfledged plovers are spotted in such a way as very

accurately to resemble the beach pebbles among which they remain for

protection.

The white-headed fruit pigeon (_Ptilopus cinctus_) is a very conspicuous

bird when taken from its native haunts. It has a pure white neck and

head, black back, yellow belly, and well-marked, deeply-curved black band

across the breast, and black wings. It is a handsome as well as very

conspicuous bird. It frequents trees that are a species of Eucalyptus in

the island of Timor, and which have very open foliage and yellowish or

whitish bark. The pigeons may be sitting motionless on exposed branches

of the tree during the glaring heat of the day. The yellow and white bark

of the tree, the deep blue sky seen through the openings of the leaves,

with the intense tropical sunlight casting black shadows of one branch

upon another, make color effects that harmonize so completely with the

color patterns of the pigeons that they are entirely invisible.[8] Here

again the complete color adaptation of the pigeons to their environment

has been accomplished through the agency of Natural Selection,—Nature

selecting those pigeons most frequently that varied most in the direction

of useful color patterns, by concealing them from their enemies, the

hawks.

There are some birds that are not protectively colored, which therefore

do not possess a local color adaptation. The common raven, which inhabits

the Arctic regions, is black instead of being white. The raven is found

as far north as any known bird or mammal. It is a powerful and fearless

bird, and needs no protective coloring; and since it feeds on dead

animals it needs no concealing coloration to enable it to secure its food.

The bright and conspicuous coloration of many birds’ eggs have often

been looked upon as a difficulty on the theory of adaptive coloration.

But Wallace thinks that a careful consideration of the subject in all

its bearings shows that in a great number of cases these colorations

are instances of protective coloration. He further thinks that when we

cannot see the meaning of the particular colors, we may suppose that in

some ancestral forms they have been protective, and, being harmless,

have persisted under changed conditions which rendered the protection

needless. He states, in illustrating the protective coloration of

eggs, that “the beautiful blue or greenish eggs of the hedge-sparrow,

the song-thrush, the blackbird, and the lesser redpole, seem at first

sight especially calculated to attract attention; but it is doubtful

whether they are really so conspicuous when seen at a little distance

among their surroundings. For the nests of these birds are either in

evergreens, as holly or ivy, or surrounded by the delicate green tints

of the early spring vegetation, and may thus harmonize very well with

the colors around them. The great majority of the eggs of our smaller

birds are so spotted or streaked with brown or black on variously tinted

grounds, that when lying in the shadow of the nests and surrounded by the

many colors and tints of bark and moss, of purple buds and tender green

or yellow foliage, with all the complex glittering lights and mottled

shades produced among these by the spring sunshine and by the sparkling

raindrops, they must have a quite different aspect from that which they

possess when we observe them away from their natural surroundings.” We

have here, probably, according to Wallace, a similar case, of general

protective harmony, to that of the green caterpillars with beautiful

white or purple bands and spots, which, though gaudily conspicuous when

seen alone, become practically invisible among the complex lights and

shadows of the foliage they feed upon.

Eggs that are not protectively colored are usually white or of some

uniform pale color, and are concealed by the birds in covered nests or in

holes in trees or in the ground. Many birds lay white eggs in open nests,

but even here the devices for concealing them are very effective and

interesting. In some cases, such as the partridge, the goat-sucker, and

others, the birds have the habit of sitting close and almost continuously

on the eggs. These birds are protectively colored. Ducks, pheasants,

and other birds have the habit of covering their white eggs with dead

leaves or other material when they leave the nest. There are some large

and powerful birds that lay conspicuous white eggs in open nests. Such

are the cormorants, herons, storks, pelicans, and others. But they guard

their nests carefully, and are able to drive away any enemies.

It is thus seen that there are many devices by which birds’ eggs are

protected, and these have all been developed through the agency of

Natural Selection. For example, those exposed eggs of timid birds that

varied most in the direction of protectively colored ones were the

ones that were most likely to be overlooked by egg-eating creatures,

and those that were least protectively colored, and therefore the most

conspicuous, were the ones most likely to be destroyed. The protectively

colored eggs being thus, in the nature of things, the ones that have

suffered less destruction from enemies, have developed into birds that

have transmitted, by heredity, their characteristics to succeeding

generations of birds’ eggs. And thus in the course of time have many of

the colorations of eggs been developed from simple to elaborate patterns.

Many snakes, frogs, butterflies, caterpillars, and so on, are colored

green in harmony with the foliage of the trees among which they live. By

being colored green these animals are more or less effectually hidden

from their enemies, such as carnivorous birds. If they were not concealed

by protective coloration they would quickly be exterminated by hungry and

greedy enemies. The green frogs are greatly protected by their colors

from their enemies, the green snakes. If they were not so colored, they

would quickly be exterminated by these snakes. Even protectively colored

as they are, large numbers of them are caught by the snakes, which are

also protectively colored and keener witted. It may be stated that the

protectively colored frogs are hunting for their food, the protectively

colored insects; but, at the same time, they try to avoid their

protectively colored enemies, the snakes, though they are often caught;

the protectively colored snakes are hunting for their food, the frogs,

but, in their green surroundings, endeavor to avoid their enemies, the

reptile-eating birds, though often unsuccessful. And so on, from day to

day, the tragedies of the animal world are enacted.

It may be well to pause a moment here and ask why it is that animals are

not completely exterminated which are thus perpetually being persecuted

and preyed upon by stronger foes. Why, for instance, are any frogs left

in a green locality, when they are so eagerly sought for by their

enemies, the snakes? What are the factors that permit the average or

normal number of them to exist in the given locality? The most important

of them have already been referred to separately, but it will be useful

to recapitulate, briefly, a few of them now. One of the most important

of them is the tendency of the frogs to increase in geometrical ratio.

In the breeding season great numbers of young are brought into the

world, thus reinforcing the depleted ranks of the adults. Another factor

is the instinctive effort of each frog to avoid its enemies, and their

protective coloration greatly facilitates their concealment and escape.

Another factor is the circumstance that the snakes also have enemies.

They must be wary and careful in hunting prey lest they be unduly

exterminated by their foes, the snake-eating animals. Then another factor

is the alternating day and night, by which the warfare, offensive and

defensive, must be periodically checked. Then again, the hibernating

season, winter, checks or rather suspends the life-destroying crusades.

It may thus be recognized that many complex factors occur to explain the

circumstance that a given locality generally contains the normal number

of individuals that constitute the species of frogs.

The complex factors at work in these relations and inter-relations of

animals and plants are so nicely balanced that with a comparatively

stable environment the number of species of the different classes of

animals may remain more or less constant. But suppose one of these

factors is profoundly diminished, such, for instance, as a great

diminution in the number of the snake-eating birds. Then the snakes,

not being much persecuted by their enemies, will be in a most favorable

environment; they will increase greatly in numbers; they will be eagerly

on the look-out for their food, the frogs; the frogs, being thus

unusually persecuted and hard-pressed, will diminish greatly in numbers.

Now, this will be a golden opportunity for the insects; their environment

becomes much more favorable. The snakes do not bother the insects, but

destroy their enemies, the frogs. Therefore, the insects increase greatly

in numbers and profoundly affect the vegetation upon which they live

and thrive. The injury to the vegetation may thus seriously affect an

altogether different class of animals—the grass-eating or foliage-eating

animals, like the ruminants.

It may thus be understood how profoundly complex are the relations and

correlations of living creatures, and how a disturbance of some of the

links in these living chains may very extensively affect the other links.

The different creatures referred to above are probably the simplest

illustrations that can be given of protectively colored animals. For

here we have green animals adapted to a green environment—the green

foliage of trees or grass. This adaptation has been brought about by

Natural Selection. Suppose, for instance, that the habitat of the

_non-protectively_ colored _ancestors_ of the frogs was for any reason

unduly crowded by their enemies, the snakes. The frogs would be hard

pressed and much persecuted. Suppose that among the young of these

frogs there were some that varied in the direction of grass-green or

leaf-green colors, while others did not so vary, or possibly varied in

the direction of even conspicuous colors, such as black or white. It is

evident that the latter groups would be easily detected by the snakes and

destroyed, while the green group would frequently escape notice and would

thrive and procreate their kind. This process being repeated generation

after generation, there would come a time when the given habitat would

contain none but frogs with a protective coloration of green. All of

these protective colorations of snakes, butterflies, and caterpillars,

as well as of frogs, are adaptations. They are illustrations of the

survival of the fittest; the survival of those best adapted to their

environment—in short, Natural Selection.

[Illustration: PLATE II.—_Kallima paralekta._ A butterfly of Sumatra

illustrating the work of Natural Selection: C, butterfly with expanded

wings (dorsal surface) which are conspicuously colored through Sexual

Selection; B, same butterfly with wings closed (ventral surface) and

presenting a close resemblance to a dead leaf, A, through the agency of

Natural Selection. B, illustrates the Protective Resemblance of an animal

to an inanimate object.]

=Protective Imitation of Particular Objects.= Insects often exhibit a

very great amount of detailed resemblance to the leaves, flowers, and

twigs of plants among which they live. Those that live on grass are

striped longitudinally, while those that feed on ordinary leaves have

an oblique striation. There is a larva of a Georgia butterfly (_Sphinx

fuciformis_) which feeds on a plant having small blue flowers and linear,

grass-like leaves. This larva has a blue head and a green body striated

like the leaves. The resemblance of the insect to its environment is very

striking. There is another species that feeds on a plant with small red

flowers situated in the axils of the leaves, and this larva has a row of

seven red spots of unequal size, which corresponds quite closely with

the size and color of the flowers. There is a caterpillar in Borneo that

resembles a piece of moss with two exquisite pink-white seed-capsules.

Its general hue is greenish, with two little pink spots on its upper

surface, and it is covered with hair. Its movements are very slow, and

when eating it withdraws its head beneath a mobile fleshy hood that it

possesses, so that its motions in feeding are not noticeable. When living

in its native haunts it is all but impossible to detect it, so completely

does it resemble the surrounding moss. Other insects resemble green or

dead leaves in all their varieties of form and color, and to show what

a great protection such resemblance affords to insects in concealing

them from view the following observation of a naturalist in Nicaragua

may be related. In that country there are armies of foraging ants that

devour every insect they can catch. Among a multitude of these ants he

observed a locust that looked very much like a green leaf. The ants, many

of them, were continually running over the body and legs of the insect

without detecting its character. In many parts of the world there are

many butterflies (_Kallima_, for example) the under surfaces of whose

wings very closely resemble dead leaves. They frequent dry forests and

are rapid flyers. They are rather large, and the upper surfaces of their

wings are quite showy, having bluish and orange colors (Plate II). It is

their habit always to settle on some twig where there are decaying or

dead leaves. In doing so it folds its wings together over the back, thus

concealing the gay upper surfaces and presenting the protectively colored

under surfaces. The resemblance to a dead leaf is much more striking from

the fact that the short tails of the hind wings just touch the branch on

which the insect rests and look very much like the stalk of a leaf. From

this stalk a dark curved line extends to the elongated tip of the upper

wings, thus imitating the midrib of a leaf. On both sides of this midrib

are oblique lines that are partly markings and partly nervures, which

give the appearance of a leaf with its veining. The head and antennæ

fit in such a way between the closed upper wings as not to interfere

with that irregular outline which is characteristic of the withered and

dry leaves. Often the closed wings are covered with small black dots

gathered into circular groups that exactly resemble the minute fungi

found on decaying leaves, and it is sometimes difficult to believe that

the insects themselves are not attacked by a fungus. Wallace states that

this wonderful imitation is most complete, and that in Sumatra he has

often seen a butterfly enter a bush and then disappear as if by magic. He

states that once he was so fortunate as to see the exact spot on which

the insect settled, but even then lost sight of it for some time, and was

able to discover it close to his eyes only after persistent and careful

search.

The curious and interesting leaf-insects of Java are veined and colored

in such a way that, with the leaf-like expansions from various parts

of the body, not one person in a dozen can detect them when they are

resting upon their food-plants right under one’s eyes. Other insects

resemble pieces of stick (Plate III), with all the minute details of

branches and knots. An eminent naturalist has stated that after being a

practical entomologist for thirty years, he was deceived by one of these

stick insects and took out his pruning knife to cut from a plum-tree

what seemed to him to be a projecting spur. This spur proved to be the

caterpillar of a geometer-moth, about two inches in length. He placed

a portion of the plum-tree on a table, and showed it to several members

of his family, designating a space of several inches in which the

caterpillar was to be found, but none of them could detect the insect

until it was pointed out to them. These protective resemblances of living

creatures to inanimate objects are beautiful illustrations of adaptation

to environment through Natural Selection.

Beautiful illustrations of protective resemblance to particular objects

are furnished by leaf-hoppers (insects) in Central America. They resemble

the thorny and prickly growths of the plants on which they presumably

live. Some of them also resemble gall growths on the plants (Plate IV).

[Illustration: PLATE IV.—Central American Leaf Hoppers resembling the

prickly and thorny growths of plants on which they presumably live.

Certain of them also represent gall growths on the plants. Protective

Resemblance. [Figures collected by Dr. L. O. Howard, from various plates

published in the Biologia Centrali-Americana.]]

=Alluring Coloration.= Besides those insects which secure protection from

enemies by their resemblance to the inert objects among which they live,

there are others whose adaptive resemblance, and therefore concealment,

is for the purpose of securing their food,—for alluring their prey. A

most interesting case of alluring coloration is that furnished by a

wingless insect of India, the mantis. Its color and form are such as to

closely resemble such a fantastic flower as the pink orchis. The insect

rests motionless among the bright green foliage, being very conspicuous

on account of its pink color, and looking so much like a flower that it

allures and captures the butterflies which settle upon it.

There is a species of spider (_Thomisus citreus_) of a creamy-white color

whose abdomen completely resembles in color and contour the unopened buds

of the flowers among which it rests. It has been seen to capture flies

that were attracted to the flowers. There is another species of spider

that looks exactly like the excreta of birds, and through this alluring

resemblance captures certain butterflies. A naturalist has related how,

in pursuing a butterfly through a jungle in Java, he was stopped by a

dense growth of bushes. Here he observed a leaf with a bird’s dropping

upon it, and sitting on this dropping was a beautiful butterfly.

Surprised at such a usually dainty and pretty butterfly seeking such

inappropriate food, he carefully approached to study the actions of the

insect. The insect permitted him to get so close that he seized it by

the wings, and to his astonishment a part of the body remained behind as

if the bird’s dropping was very adhesive. He touched the dropping to see

if it really was sticky, and found that his eyes had been deceived and

that what he took for the excreta of a bird was a most artfully colored

spider, lying on its back with its feet crossed and depressed closely to

the body. The spider had been firmly holding the butterfly.

[Illustration: PLATE III.—Caterpillar, B, of a Geometer Moth

(_Prochœrodes transverrata_) on the stem of a plant (_Ailanthus_), A.

Illustrating Protective Resemblance.]

A very pretty illustration of alluring coloration is furnished by the

common spider (_Misumena vatia_), which is often found spread out upon

the yellow heart of an ox-eyed daisy and in like position upon Coreopsis

(Plate V). It so closely resembles the flower upon which it lurks that

the ordinary observer might well fail to notice its presence. The

coloration facilitates the taking of prey and protects the creature from

the assaults of enemies (Rev. H. C. McCook). It will be observed from the

illustration that the flower is mostly yellow with some red; the same

is true of the spider. This case is a very instructive illustration of

alluring coloration by which the insect has become a living bait for

entrapping butterflies and other insects. Nature has brought about this

complete color adaptation, from the variations of myriads of spiders.

For those ancestral spiders that varied most in the direction of color

adaptation to their environment were less often shunned by shy insects

which could serve as prey. The spiders with useful color variations

would thus most likely secure an abundance of food, and, thus living,

transmit to many of their offspring their useful variations; while those

with inharmonious, and therefore harmful, variations would not be so

able to deceive their food insects, and would thrive poorly or starve

altogether. This is another illustration of the survival of the fittest,

the selection by nature of those best adapted to or in harmony with the

environment; in short, it is Natural Selection.

Wallace, from whom many of these illustrations are taken, says that to

many persons it will seem impossible that such beautiful and detailed

adaptations and resemblances—and these are only samples of thousands

that occur in all parts of the world—can have been brought about by the

preservation of fortuitous useful variations. Yet this will not seem so

surprising, continues Wallace, if we keep in mind the facts of the rapid

multiplication of animals, the severe struggle for existence, and the

constant variability of these and all other organisms; and, further,

that we must remember that these delicate adjustments are the result of

a process, Natural Selection, which has been going on for millions of

years, and that we now see the small percentage of successes among the

myriads of failures. “From the very first appearance of insects, for

instance, and their various kinds of enemies, the need for protection

arose and was usually most easily met by modifications of color. Hence

we may be sure that the earliest leaf-eating insects acquired a green

color as one of the necessities of their existence; and, as the species

became modified and specialized, those feeding on particular species

of plants would rapidly acquire the peculiar tints and markings best

adapted to conceal them upon those plants. Then, every little variation

that, once in a hundred years perhaps, led to the preservation of some

insect which was thereby rather better concealed than its fellows, would

form the starting-point of a further development, leading ultimately to

that perfection of imitation in details which now astonishes us.” So it

is with the beautiful color adaptation of birds, mammals, lizards, and

other animals. There is a lizard (_Phrynocephalus mystaceus_) inhabiting

certain sandy districts in Asia, whose body is protectively colored and

some of whose mouth-parts have alluring coloration and form. The general

surface harmonizes with the sand in which it is found, while the skin

at each angle of its mouth is of red color and so folded as to closely

resemble a little red flower which grows in the sand. The lizard, being

thus in harmony with its surroundings, resembling the sand and the

flowers, is hidden from its enemies, the reptile-eating creatures. But at

the same time insects, being attracted by what they take to be flowers,

approach the lizards and are thus captured, being allured to their

destruction.

[Illustration: PLATE V.—A spider (_Misumena vatia_) lurking for prey on

the center of a flower (_Coreopsis_). Illustrating especially alluring

Coloration (for attracting prey), but also Protective Resemblance

(against enemies). Reproduced from “American Spiders.” By courtesy of

Rev. Henry C. McCook.]

=Warning Coloration.= Many animals possess color patterns that render

them very _conspicuous_ in their environment. It is a very interesting

fact that most of these creatures are the possessors of some deadly

weapons, as poison-fangs or stings, or that they are very disagreeable

and unpalatable food for other animals. Warning colors are most abundant

and best developed among insects. A family of butterflies (Heliconidæ),

in tropical South America, possesses very pronounced and conspicuous

color patterns, so that they are easily seen in their native haunts. Many

of them have deep blue-black with vivid red, white, and yellow spots and

bands, totally unlike those butterflies in the same locality that are

protectively colored and palatable. Their bodies have juices that exhale

a powerful odor. If one kill them by pinching the body, a liquid exudes

that stains the fingers yellow and leaves an odor on them that can be

removed only by repeated washing of the hands. There is a great deal

of evidence to show that this odor is very offensive to insect-loving

animals. Protectively-colored butterflies fly with great rapidity and are

very wary and seek concealment; while the butterflies with conspicuous

colors fly slowly, and do not conceal themselves, as if conscious that

they have no enemies.

Many caterpillars have gay and conspicuous colors and do not conceal

themselves. Bates noticed one in South America four inches long, striped

across the body with yellow and black bands, and with bright red head,

tail, and legs. It could be seen by any one who passed by, even at a

distance of many meters. All of these conspicuous and brightly-colored

caterpillars are unpalatable, and are refused as food by insect-eating

creatures.

Grasshoppers and locusts generally possess green protective tints and

are very palatable, but in tropical regions there are many species most

gaudily decorated with blue, red, and black colors. They are inedible and

are invariably rejected as food by lizards and birds.

A spider whose bite is exceedingly poisonous is found in Queensland. Its

bite will kill a dog, and produces serious illness in man, with agonizing

pain. It is black with a bright red patch on the middle of its body. This

warning coloration is so conspicuous that even the spider-hunting wasp

avoids it.

In all parts of the world frogs are usually protectively colored with

browns or greens; and little tree-frogs are either curiously mottled to

imitate dead leaves or bark, or they are green like the leaves they rest

upon. These protectively colored frogs are always eagerly sought after

by snakes and other enemies. But there are some frogs that are very

conspicuously colored and that hop about with impunity, being avoided by

the snakes and birds of prey. Such is a little frog in Nicaragua which

with its “scarlet vest and stockings of blue” is very conspicuous in its

native haunts.[9] Such, also, is a small toad found in South America,

which is colored a bright vermilion and an intense black, which crawls

about in the sunshine over the sands of arid places. Both of these

animals are altogether avoided by the frog-eating creatures, because they

have disagreeable properties that make them inedible.

In tropical America the very poisonous snake Elaps (Plate VI) is found

abundantly. Its style of coloration is very conspicuous and one that

does not occur in any other group of snakes, consisting alternately of

rings of red, black, and yellow, or red and black of varying width and

arranged in different patterns. Snake-eating birds and mammals have

learned, through hereditary experiences, to avoid these snakes with gay

livery because they are poisonous and therefore dangerous. In Plate VII

the conspicuously-colored black and yellow salamander is an animal with

warning coloration. It is inedible and avoided by carnivorous birds.

These warning colorations have been evolved through the Natural Selection

of fortuitous useful color variations in the ancestors during the

Geologic Ages.

[Illustration: PLATE VI.—Illustrating Warning Coloration (_Elaps_) and

especially Mimicry (_Erythrolamprus_). Elaps is a very poisonous reptile

and Erythrolamprus is harmless. Reproduced (and modified) from Romanes’s

“Darwin and after Darwin.” By courtesy of the Open Court Publishing

Company.]

=Mimicry.= Protective resemblance of a harmless animal to another

of a different species that is harmful is known as mimicry. Mimicry

is bound up with and altogether dependent upon warning coloration.

Some beetles are protected by having integuments, etc., of very great

hardness. Several genera of weevils are in this way saved from attack

by insect-loving birds. These weevils are often closely imitated in

appearance by softer and more eatable species of different genera from

the weevils. Wasps and bees are often mimicked by insects of other orders.

Insectivorous birds are very active in hunting out the edible beetles

(Longicornia), and everywhere in tropical regions these beetles so

closely resemble other insects which are avoided by the birds that the

longicorns are very frequently avoided and thus protected.

In tropical America many butterflies (Heliconidæ) are found that possess

warning coloration. They possess an offensive taste and odor which

almost entirely exempt them from the attacks of insect-eating animals.

The insectivorous birds have learned, by transmitted experiences

(heredity) to avoid the Heliconidæ. It is an interesting fact that in

the same locality with these distasteful butterflies are other species

that are very palatable to insectivorous creatures; but they so closely

resemble the non-edible species that the birds pass them by, not

recognizing their character.

There are some cases of mimicry among birds. There is a genus of large

honey-suckers known as friar birds found in the Malay Archipelago. They

are noisy and powerful birds which go in small flocks. They have sharp

beaks which are long and curved, and also powerful grasping claws. They

are perfectly able to defend themselves, often driving away such birds

of prey as hawks and crows when they approach them too closely. In the

same environment are weak and timid birds known as orioles, which trust

chiefly to their retiring habits and concealment for protection. The

orioles, although an entirely distinct species from the friar birds, very

closely resemble the latter. In each of the great islands of the Malayan

Archipelago there is a distinct species of friar birds, and always in the

same locality is a species of oriole that exactly mimics it. The separate

species often look so thoroughly alike that competent naturalists, prior

to a very close examination, have considered them as belonging to the

same species.[10]

The most remarkable cases of mimicry are those in which poisonous snakes

are mimicked by harmless ones. There is an egg-eating snake in South

Africa that possesses neither teeth nor fangs and is not poisonous. It

very closely resembles the poisonous Berg adder. When alarmed it still

more closely resembles the adder by the habit of flattening its head and

darting forward as if to strike an enemy, hissing at the same time.

[Illustration: PLATE VII.—An Amphibian (_Salamandra maculosa_)

illustrating Warning Coloration.]

In tropical America, in the localities where the poisonous genus Elaps

is found so abundantly, are several genera of harmless snakes of other

families, some species of which so closely resemble or mimic the

poisonous species that they are distinguished from them with difficulty.

The peculiar color patterns of the poisonous snakes serve as warning

colors to snake-eating mammals and birds. The mimicking snakes by flying

these danger-flags are protected.

In Plate VI Elaps is the venomous snake, and illustrates warning

coloration; Erythrolamprus is the edible, non-venomous reptile that has

acquired, through Natural Selection, a protective resemblance to Elaps,

and illustrates mimicry. At the first glance these two snakes look

very much alike; but a closer inspection will show that the detailed

color patterns differ in the two cases. Elaps, though a very poisonous

snake, has the reputation of not being venomous. This error has probably

originated from the fact that it has a gentle disposition and mild

temper, and also from the fact that no doubt it is frequently mistaken

for the mimicking non-venomous species.

Both warning coloration and mimicry are interesting illustrations of

adaptation to environment through Natural Selection; for the myriads

of ancestral forms were continually giving slight variations in color

patterns, some of which were useful to the creatures and others harmful.

Those that harmonized mostly with the environment gave their possessors

an advantage in the fierce struggle for life,—the struggle for food and

place and safety; and procreating their kind age after age, led to the

perfection of mimicry as we behold it to-day.

=Coloration as Recognition Marks.= In gregarious animals, whether

herbivora or carnivora, and whether mammals or birds, a ready recognition

of their own kind at a distance, in the dim twilight, or during rapid

motion, is of the greatest use, and probably often leads to the

preservation of life. Gregarious animals will not usually permit a

stranger in their midst. So long as these animals keep together they are

generally safe from enemies; but a single animal straggling off by itself

may become an easy prey to enemies. In such cases it is of the highest

importance to an animal that it should have every facility for quickly

discovering its companions at any distance within the field of vision.

Also to the young and inexperienced of each herd some means of easy

recognition is of vital importance. Recognition marks also enable the

sexes to identify their kind readily. The necessity for easy recognition

probably is at the basis of the bilateral symmetry in the coloration of

animals.[11] In the struggle for existence those gregarious creatures

that have the best color patterns for recognition marks are the most

likely to get the scattered members of a herd together with the greatest

rapidity, and thus to save them from their enemies. The surviving

members, transmitting their useful variations in recognition marks

to their offspring, are thus able in the course of ages to bring into

existence pronounced color patterns. Thus Natural Selection can account

for much of the coloration in animals known as recognition marks.

One or two illustrations of recognition marks in creatures will be

sufficient. A rabbit when alarmed and fleeing to its burrow displays a

conspicuous, upturned white tail. The rest of the body is protectively

colored. This conspicuous white surface of the upturned tail is a signal

flag of danger. The rabbit mostly feeds during moonlight nights, or

soon after sunset. The white upturned tails of alarmed rabbits serve as

signals and guides to the feeble and young, and also to those at a more

remote distance. Thus a number of rabbits, each following the one or two

in front, are all able in the quickest manner to reach a place of safety.

The spring-bok has a white patch on the face and one on the sides. It

also has a curiously well-marked white stripe above the tail. When

the animal is at rest this last-mentioned white stripe is very nearly

concealed by a fold of skin, but when it is in motion it comes into

full view, like the upturned tail of the rabbit, and serves as a guide

to friends. There are some animals inhabiting the Arctic regions

that are not white,—they are not protectively colored. Such is the

musk-sheep. This animal, though living in Arctic regions, is yet brown

and conspicuous. Its safety depends upon its association in small herds.

It is gregarious. Therefore it is of much more importance to this animal

that it should be able to quickly recognize its companions at a distance

than that it should be protectively colored and so concealed from its

enemies. So long as they keep together in herds they are abundantly able

to protect themselves. This is an exception to the rule of local color

adaptation that proves the rule.

=Sexual Selection.= Among most backboned animals it is the rule that both

sexes should be alike in color. This is especially true among the fishes,

reptiles, and mammals. But in birds the diversity of sexual coloring is

very frequent. It is among this class, therefore, that Sexual Selection

can best be studied. One of the most fundamental characteristics of

birds is the greater conspicuousness of coloration in the males. In the

tropical regions especially are found the most striking examples of

divergence in sexual color patterns. In humming birds, the pheasants,

the peacocks, chatterers, tanagers, and birds of paradise, the females

are exceptionally dull-colored and plain, while the male birds are

gorgeously colored and conspicuously attractive. The male birds of

paradise, for instance, are not only brilliantly colored, but also have

remarkable gorgets, plumes, and crests; whereas the female paradise birds

are without these decorations and as plain as our thrushes in their

ornamental plumage (Plate VIII).

The splendor of plumage which characterizes the male pheasants is

entirely wanting in the females. The intense crimsons and pure whites,

the gorgeous purples and blues of the male chatterers contrast strikingly

with the dull browns or olive greens of the females. The sober hues of

the females have been accentuated by Natural Selection.[12] When the

females were brooding on the eggs in their nests, those of them that had

varied in the direction of conspicuousness would most readily be detected

by their enemies, the hawks, and would be exterminated; but those that

had quiet and dull ornamentation would most frequently escape discovery,

and would pass successfully through the brooding season; thus living and

transmitting their color patterns to posterity,—Natural Selection ever

eliminating the conspicuous and preserving the sober-hued,—in the course

of time the dull ornamentation of the females would become more and more

pronounced.

[Illustration: PLATE VIII.—Male and Female Paradise Birds (_Paradisea

minor_). Illustrating the effects of Sexual Selection. The upper figure

(male bird) is much more beautiful than the lower (female bird).]

But another factor has been at work in accentuating the marked

differences in the ornamentation of the sexes. This factor is Sexual

Selection.[13] In the brooding season there is an intense rivalry

among the males for the possession of the females. Among them the art

of courting has become, indeed, one of the fine arts. The male birds,

like the males of almost all animals, have stronger passions than

the females, and with rarest exceptions are much more eager than the

females. In courtship they display their adornments and accomplishments

most zealously before the females; they strut around them in most eager

courtship and pompous vanity, displaying the utmost rivalry. The males

charm the females in various ways, such as dancing, or performing

fantastic antics either in the air or on the ground; and then again by

most melodious song. After man, the female birds appear to be the most

æsthetic of all animals; therefore, those male birds that are the most

pleasing songsters, or the most attractive in their dances and fantastic

performances, are the ones selected by the females for mating. In this

selection the female birds have paid the minutest attention to fleeting

fashions in strut and dance, in form and color,—the progeny of those

males that have been selected by the females for mating, transmitting

the inherited peculiarities of the parents, have tended more and more to

stamp as fixtures these fleeting fashions, and in this way the males have

become endowed with all sorts of decorations and accomplishments.[14]

Thus have been produced in them the many forms of topknots, wattles,

combs, plumes, and feathers elongated and springing gracefully from many

portions of the body; also the naked skin of the head and the beak,

frequently colored gorgeously. The feathers, through this means, are

often most beautifully tinted in charming patterns. As bearing upon this

theory of Sexual Selection, it can be observed that birds pay the closest

attention to the songs of each other. A bullfinch, for instance, had

been taught to pipe a German waltz, and in doing so was a most excellent

performer. He was placed in a room where there were kept some eighteen

canaries and linnets, and immediately commenced producing his melodies.

The birds all ranged themselves on the sides of their cages nearest

the performer, listening to his singing with the greatest interest.

Undoubtedly this singing is most often a matter of courtship; the female

finch selects that one out of a hundred males whose notes charm her the

most; the female canary always chooses the best singer. The soft cooing

of pigeons and of turtledoves is a matter of courtship. In the breeding

season there is the most intense rivalry between the males in singing; a

bird will sometimes sing until he drops down almost dead.

That female birds exercise choice in mating was believed by Audubon.

He describes how a woodpecker hen was followed by six gay suitors who

continued to perform strange antics until a marked preference was shown

for one of them. A study of the Australian bower-birds illustrates both

the courting antics of the males and the exercise of choice by the

females. These birds build bowers which are sometimes quite large. That

of one species is raised on a thick platform of sticks and is nearly four

feet in length and eighteen inches in height. These bowers are built

on the ground, and are for the sole purpose of courtship, since the

nests are formed in the trees. They are highly decorated with leaves,

berries, feathers, shells, and kindred objects. Both sexes assist in the

erection of the bower, although the male bird is the principal worker.

The bower-constructing instinct is so strong that it is practiced even

in confinement. A naturalist in describing the habits of some satin

bower-birds kept in an aviary says that at times the male will chase the

female all over the aviary, then go to the bower, pick up a gay feather

or a large leaf, utter a curious kind of note, set all his feathers

erect, run round the bower, and become so excited that his eyes appear

to start from his head. He continues opening first one wing and then the

other, uttering a low, whistling note, and, like the domestic cock, seems

to be picking up something from the ground, until at last the female goes

gently toward him and the wooing is completed.

Instances of love dances may be taken from all classes of the animal

kingdom. “Mr. Peckham has described a very interesting love-dance by a

certain species of spider (_Saitis pulex_). He placed a male in a box

with a female. As soon as the former saw the latter, about twelve inches

away, he became excited and at once moved towards her; when some four

inches from her, he stood still, and then began the most remarkable

performances that an amorous male could offer to an admiring female.

She eyed him eagerly, changing her position from time to time so that

he might always be in view. He, raising his whole body on one side by

straightening out the legs, and lowering it on the other by folding the

first two pairs of legs up and under, leaned so far over as to be in

danger of losing his balance, which he maintained only by sidling rapidly

towards the lowered side. The palpus, too, on this side, was turned back

to correspond to the direction of the legs nearest it. He moved in a

semicircle of about two inches, and then instantly reversed the position

of the legs, and circled in the opposite direction, gradually approaching

nearer and nearer the female. Now she dashes towards him, while he,

raising his first pair of legs, extends them upward and forward as if to

hold her off, but withal slowly retreats. Again and again he circles from

side to side, she gazing towards him in a softer mood, evidently admiring

the grace of his antics. This is repeated until are counted one hundred

and eleven circles made by the ardent little male. Now he approaches

nearer and nearer, and when almost within reach whirls madly around and

around her, she joining and whirling with him in a giddy maze. Again he

falls back and resumes his semicircular motions, with his body tilted

over; she, all excitement, lowers her head and raises her body, so that

it is almost vertical; both draw nearer; she moves slowly under him, he

crawling over her head, and the mating is accomplished.”[15]

In addition to that form of Sexual Selection where the female chooses a

mate from among a number of competing males, and which may be designated

_preferential mating_, there is another form of selection in which the

males fight with one another for the mastery and the possession of the

females. Among the higher mammals it is a very general fact that the

males fight together for the possession of the females. This leads,

especially in polygamous animals, to the better armed or stronger

males becoming the parents of the next generation, which inherits the

peculiarities of the parents. Thus the offensive weapons and the vigor

of the males are continually increased, resulting in the antlers of the

stag, the tusks of the boar, the fighting instinct and spurs of the

gamecock, and the horns and strength of the bull. Even mammals that are

not specially armed fight to the death for the possession of the females,

such as beavers, moles, squirrels, and hares. Almost all male birds are

especially pugnacious during the breeding season. Battles have been

observed in such different groups as ducks, finches, woodpeckers, humming

birds, and waders. Among fishes deadly battles occur between the males

of sticklebacks. Also the males of salmons engage in deadly contests;

among reptiles fighting occurs among the male tortoises, crocodiles, and

lizards. Spiders and many butterflies often fight for the females. Thus

Sexual Selection through the _law of battle_ occurs widely throughout

the animal kingdom. This form of Natural Selection greatly increases the

vigor and fighting power of male animals; for, in every case, the weaker

males are either driven away, killed or wounded, and the field is left to

the most vigorous for procreating their kind.

The male stickleback is a little fish that builds its nest among the

weeds, weaving the material together by a secretion from its kidneys.

It is a very passionate little animal, and is exceedingly pugnacious in

relation to its male rivals. The battles of the males are often very

desperate. The combatants fasten tight to each other for a time, tumbling

over and over again, until they appear to be completely exhausted. The

males of the rough-tailed stickleback, when fighting, swim round and

round one another, endeavoring to pierce each other with their raised

lateral spines. In fighting they are perfect little furies, and their

bite is very severe. Their lateral spines are used with such fatal effect

during a battle that a male has been observed to rip open his opponent,

so that the latter sank to the bottom in a dying condition. The females

are very peaceful. When they come out of their hiding-place and view the

nest that the male has made he is mad with delight.

The male salmon is as pugnacious as the little stickleback. Two males

have been known to virtually battle with each other all day long. In

breeding ponds the males can be seen constantly fighting and tearing one

another on the spawning beds, and so many are injured in consequence

that they may be seen swimming near the banks in a state of exhaustion,

many of them apparently dying.

Among birds the law of battle holds as well as the law of preferential

mating. During the breeding season they are exceedingly pugnacious. The

humming birds, the smallest of any, are among the most quarrelsome. Two

males rarely meet without a fierce fight on the wing.

Darwin gives the following illustration of the invincible courage and

fighting instinct of the gamecock. One of these birds had both of its

legs broken in a cockpit, and its owner made a wager that if his legs

could be supported with splints and bandages so that he could stand

upright he would keep on fighting. This was accomplished, and the bird

fought on with dauntless courage until he received a death stroke.

It is probable that even with the most pugnacious species of birds

the pairing does not depend alone on the courage and strength of the

male, for these males are usually decorated with various ornaments.

Furthermore, these decorations during the breeding season often become

more brilliant, and are eagerly displayed before the females. Darwin

states that twenty or more males of the _Tetrao cupido_ (species of

grouse) will assemble at a particular spot, keeping up a tremendous

chattering and strutting about. At the first response from a female the

males take to fighting furiously, and the weaker are vanquished. Both the

victors and the vanquished pay court to the female, so that the latter

must make a choice or the battle is renewed. Here we have the combined

action of selection through battle and by preferential mating.

Among mammals the males win the females much more through fighting than

through the display of charms. In the breeding season the most timid

male animals, which are not even supplied with any special weapons for

fighting, engage in the most desperate conflicts. Two male hares have

been seen to fight until one was killed. Those male mammals which are

provided with special weapons for fighting enter into the fiercest and

most deadly conflicts. The wild male elephant, during the period of love,

is one of the fiercest fighters in the world. Lions engage in terrible

battles, and a young lion dare not approach an old one.

Male seals fight most desperately during the breeding season, using

both their claws and teeth. The conquerors appropriate the females and

transmit their qualities to their offspring.

These are all cases of the survival of the fittest; of the survival of

the males best adapted to the exigencies of their surroundings—in short,

additional illustrations of Natural Selection.

=Insect Selection.= Flowers usually consist of several parts, such as

the stem, the calyx composed of green sepals, the corolla formed of

attractively colored petals, the stamens, the pistil, and finally of

certain nectar-forming organs situated at the base of the last-named. The

upper portions of the stamens are known as antheridia. The free extremity

of each pistil is called a stigma, the intermediate stalk is the style,

and the base is the ovary. The flowers constitute the sexual parts of the

plant. The stamens are the male elements and the pistils are the female

structures. In some species of plants the flowers possess the stamens

but not the pistils,—they are male flowers; in others the flowers have

pistils but not stamens,—they are female flowers. In still other plants

the flowers possess both the male and the female structures, and are

therefore bisexual or hermaphroditic flowers. The ovary of a pistil has a

number of cells in it called ovules (female germ cells); the antheridia

on the stamens have cells in them called pollen (male germ cells). When a

pollen cell is carried by any means to the stigma of a pistil, it sends

down through the latter a tubular prolongation by which the nucleus and

protoplasm of the pollen cell unite with the nucleus and protoplasm of

the ovule; so that we now have a fertilized egg,—the germ of a new plant.

This fertilized ovule by repeated cell multiplication can grow into an

adult plant.

There are two agencies by which the pollen is carried to the pistil;

first, by the wind, and, secondly, by insects (or, occasionally, by

humming birds). Flowers that are fertilized through the agency of insects

are the most beautiful in existence, displaying all the varied hues and

gorgeous patterns that are found in the organic world. On the other hand,

the flowers which are fertilized through the agency of the wind are

incomparably less beautiful than those of insect production, and they do

not secrete sweet juices or nectar. The earliest flowers in geologic time

consisted only of those essential portions, the stamens and pistils, and

had no colored whorl of petals within another colored whorl of sepals.

The poorly developed nectaries secreted only small quantities of honey.

The food-seeking insects visited these primitive flowers for the pollen

and nectar, even as they do now. The nectaries in the plants were so

situated that the insects could not get at the honey without rubbing off

some of the pollen and carrying it to other plants which it visited for

honey. In this way the nectar-seeking insects would often carry pollen

from the flower of one plant to the stigma of another plant’s flower,

and thus secure cross-fertilization. Undoubtedly often the stigma of a

plant’s flower was fertilized by pollen from the same flower; but it is

known that seeds produced by cross-fertilization are much more vigorous

and hardy than those produced by self-fertilization in the same flower;

therefore, those plants that varied most in the direction best adapted

for securing cross-fertilization would have decided advantages in the

struggle for existence. Their fertilized eggs (seeds) would be most

vigorous, and would most readily secure nutriment and withstand adverse

circumstances. The variations of the primitive flowers most calculated

to secure cross-fertilization would be those that made the flower more

conspicuous by the appearance of bright colorations and delightful odors,

such as would attract the attention and visits of nectar-seeking insects.

These variations being transmitted, by heredity, to the descendants and

accentuated as the ages passed would ultimately lead to all the wonderful

adaptations of flower and insect structure to one another that are found

in nature.

[Illustration: PLATE IX.—Two Primrose flowers (_Primula elatior_). A,

with a long style; B, with a short one. Vertical sections have been made

through the flowers.]

Natural Selection has acted in a double manner here, preserving those

flowers that had the most delightful odors or the most conspicuous and

enticing colorations and patterns; and at the same time selecting those

insects that varied most in the direction of keenness of scent, acuteness

of vision, and color perceptions. It will be instructive to briefly

describe the fertilization of the primrose and an orchis by bees. The

_primrose_ (Plate IX) has two sorts of flowers that are never found on

the same plant; one has stamens high up the corolla tube and a short

style with its stigma below the anthers; the other has the stamens far

down the tube and a long style with the stigma above the anthers. In

both cases the nectar is at the very bottom of the corolla tube. When a

bee visits a short-styled flower, it extends its proboscis to the bottom

of the tube and in withdrawing it brings away some pollen cells at its

base. If it should next visit another short-styled primrose, it would not

fertilize it because the pollen on the base of the proboscis could not

reach the stigma; it would only gather still more pollen. But when it

visits a long-styled primrose, it will necessarily deposit some pollen

cells on the stigma, inasmuch as that is at the commencement of the

corolla tube. If the bee should first visit a long-styled form of flower,

it will carry off pollen on the tip of its proboscis, and when it visits

a short-styled primrose will deposit the pollen on the stigma.

In the _orchis_ the stigma is placed below the anther. The stigma is

in the front part of the flower and at the base of the lip, the latter

being prolonged into a long tube full of nectar. The stigmatic surface

is composed of very viscid matter. A bee when seeking the nectar pushes

its head against the anther, and in so doing detaches the two sticky

glands to which the club-shaped masses of pollen cells are attached

(Plate X). It carries these away on the front part of its head. So long

as the masses of pollen cells remain erect on the bee’s head, they cannot

reach the stigma of any other orchis that it visits. As the sticky glands

on the head contract, the pollen masses incline forward and become

horizontal, so that they touch the next sticky stigma that is visited.

The greater stickiness of the stigma detaches the pollen masses from the

bee’s head, so that the flower is fertilized. It takes so long for the

pollen masses on the bee’s head to assume the horizontal position that

the insect has visited all the flowers on one orchis and then visits

another plant. By this time the pollen masses are in the proper position

(horizontal) to fertilize the flowers of another plant. In this way

cross-fertilization is secured and the vigor of the plant maintained.

It may now perhaps be appreciated how intimate are the relations between

the form and habits of insects and the structure and coloration of

flowers. Our standards of beauty have largely been created for us through

Insect and Sexual Selection. If insects had not been developed on the

earth, plants would not be ornamented by beautiful flowers, but would

show only such poor and inconspicuous ones as are found on grasses, on

oak, ash, and fir trees. Grant Allen has beautifully written that, “while

man has tilled only a few level plains, a few great river-valleys, a few

peninsular mountain slopes, leaving the vast mass of earth untouched by

his hand, the insect has spread itself over every land in a thousand

shapes, and has made the whole flowering creation subservient to his

daily wants. His buttercup, his dandelion, and his meadowsweet grow

thick in every English field. His thyme clothes the hillside; his heather

purples the bleak grey moorlands. High up among the Alpine heights his

gentian spreads its lakes of blue; amid the snows of the Himalayas his

rhododendrons gleam with crimson light. Even the wayside pond yields

him the white crowfoot and the arrowhead, while the broad expanses of

Brazilian streams are beautified by his gorgeous water lilies. The insect

has turned the whole surface of the earth into a boundless flower-garden,

which supplies him from year to year with pollen or honey, and itself in

turn gains perpetuation by the baits that it offers to his allurement.”

[Illustration: PLATE X.—Illustrating the fertilization of an orchis by an

insect. A, represents a section of the flower and shows a bee standing on

the flower’s lip with its head touching the sticky portion of the pollen

masses; C, shows the pollen masses stuck to the bee’s head and erect; B,

shows the pollen masses horizontal, the proper position to leave them

detached from the bee by the sticky stigma.]

Natural Selection is the great agency that accounts not only for the

color patterns and forms of living creatures, but also for the great

majority, if not all, of the useful characteristics of organic creatures,

including their internal organization.

One more illustration of the power of Natural Selection may be given,

outside of the subject of color patterns. It may be interesting to

readers who are not familiar with the elements of zoölogy to know that

whales are not fishes but mammals. They belong to the same class that

man does. The embryology of a whale reveals that it is descended from

ancestors that were _land_ mammals, and that these mammals had a scanty

covering of hair, teeth of different shape, broad tails like beavers,

short fore and hind legs, and well developed sense organs. The olfactory

organ was especially well developed. It is probable that these ancestors

of the whale lived in marshy districts and were omnivorous, eating both

plant and animal food. They sought their food in both shallow water and

in swamps. As the conditions of life became more and more unpropitious

on land, they were slowly modified through the ages under the action of

Natural Selection into creatures somewhat like dolphins. At first they

lived in fresh water, but finally they found their way into the sea and

became the rulers of the ocean, from which the giant sea reptiles of

earlier epochs had vanished. Hence are explained the adaptive changes

of structure: the fore-limbs were modified into flippers enclosed in a

fin-like sac, but retaining the bones corresponding to like structures

in other _mammals_, as in the arm of man, the wing of the bat, and the

fore-leg of the horse. Traces of the hind legs may be detected in a few

species; the tail, which acted as a powerful swimming organ, became

divided into two lobes; the head became fish-like in shape; the seven

bones of the neck, common to most mammals, grew together; the skin became

hairless; and the teeth, which appear in the young of the true whale, but

are never cut, gave place to hanging fringes of whalebone, in the meshes

of which the animal entangles the minute organisms it feeds upon.[16]

ISOLATION OF VARIETIES IN NATURE.

The following analysis of Isolation will be useful:

{ Preferential Mating.

{ Sexual. {

Isolation { { Cross-Sterility.

{ Geographical.

It has been stated on an earlier page that the commingling of diverse

hereditary units accomplished through the fertilization of an ovum by a

spermatozooid is the source of many variations in the offspring. In this

fertilized ovum the complexity of chemical substance, and, therefore,

the complexity of inheritance, gives instability to the embryo, and

thus produces variations in the offspring. In this embryo there is

a struggle among the hereditary units,—a struggle among the various

qualities inherited from both sides,—and a survival of the fittest, a

veritable intra-cellular Natural Selection. It is a well-established

law in biology that the union of germ cells of very closely related

individuals, that is, of consanguineous individuals (in and in breeding)

leads to less vigorous and variable offspring, and the parents are

less prolific; while the commingling of diverse heritages by the union

of germ cells from individuals belonging to strong but different

varieties leads to vigorous and quite variable offspring. The union of

such individuals is also most prolific. On the other hand the union of

individuals belonging to very diverse varieties becomes less and less

prolific until the cross-sterility of species is reached, although there

are many exceptions to this rule of cross-sterility. The individuals

of a species living in a state of nature are constantly varying. With

every generation trivial variations take place in all directions and

of all kinds. But these variations are all funded in the common stock,

for the varieties freely mingle among one another and cross-breeding

is constant not only between them, but with the parent stock. Of the

variations that are constantly taking place some are advantageous to

the creatures, some are disadvantageous, while many are neutral, being

neither useful nor harmful. Natural Selection is ever alert, selecting

the advantageous variations and eliminating the disadvantageous ones.

The neutral variations are not touched by Natural Selection; among these

intercrossing of varieties probably affords Nature an opportunity to

make almost endless combinations, some of which might be useful to the

animals, and others harmful, and in either case would come under the

influence of Natural Selection. The commingling of diverse heritages

due to the union of more or less pronounced varieties of the species in

nature not only leads to a funding of varietal characteristics, but also

increases the instability of the offspring, augmenting their plasticity,

so that more numerous and diverse variations take place. According to

Romanes the reproductive organs are among the most variable in the body.

Of the numerous variations taking place in the individuals of a species

under nature, some, therefore, affect the reproductive organs in such a

way that certain of these individuals are cross-fertile with one another,

but _cross-sterile with other varieties and with the parent stock_. This

interesting and very important kind of variation is known to occur in

some individuals of the human species. It is well known that a man and

woman have been cross-sterile with one another, being unable to have

children; yet when separating and mating with others they have both been

cross-fertile, families being reared by both of them.

Variations are commencing species; isolated variations diverge more and

more into distinct species. This fact, then, of the occurrence in nature

of variations in some of the individuals of a species by which they are

cross-fertile with one another, but cross-sterile with other varieties

and with the parental stock, shows that Nature has a most effectual means

by which varieties may be isolated from one another,—just as effective

means as man, the breeder of varieties of domestic animals, possesses

in isolating these domestic varieties by physical barriers, as fences,

etc. Cross-sterility, therefore, in nature, is a most effective sexual

barrier. The special form of it under consideration is what Romanes has

called Physiological Selection. The varieties that are isolated by this

sexual barrier have got to run the risks of in and in breeding, which

Darwin has shown occur in domesticated animals, but which Wallace thinks

are much less in a state of nature.

Another very important mode of Nature for isolating varieties is that

which arises out of the instinctive preferences of animals. There is a

tendency for like to breed with like where varieties are formed. The pale

and dark colored herds of fallow deer in the Forest of Dean have never

been known to interbreed. In the Falkland Islands all the cattle are

known to have descended from the same stock. Here there are differently

colored herds of cattle, and those cattle of the same color will

interbreed with each other, but not with individuals outside their own

color-caste, as Morgan expresses it. When two flocks of heath sheep and

merino sheep are mixed together, they do not interbreed. This isolation

of varieties by instinctive preferences for those individuals with

like color patterns may be spoken of as _preferential mating_ through

_recognition marks_. A very obvious mode of isolation in nature is by

geographical barriers (including migration). In treating of environment

we learned that during the geologic ages of the globe, the physical

geography and climate have repeatedly changed. A very cold temperature, a

mountain chain, a body of water, a stretch of desert land, may completely

prevent interbreeding, on either side of the barrier, between the

individuals of a species.

It should now be understood that Nature, like man, produces divergences

from the parental stock, and isolates them by various effective

agencies. Natural Selection ever carefully watches over these processes,

eliminating the unfit variations and selecting the useful ones.

Variation, Heredity, Environment, Isolation, and Natural Selection,

having been acting and reacting through the ages, have produced, from a

common parental stock, all the innumerable divergent and adaptive forms

of living creatures that can be traced through the geologic strata to

those on the surface of the earth to-day; have produced, therefore, _man_

also as the inflorescence of the topmost branch of the tree of life.

SECTION VI.

EVOLUTION OF MAN.

The detailed study of the development (ontogenetic and phylogenetic)

of man is so vastly intricate and extensive a subject that it will be

impossible in a work of this character to do more than refer to it in

brief outline. One of the best and simplest methods to approach the study

of such a subject is to acquire some idea of the _development of the

frog_.

The fertilized frog’s egg, which is the starting point in the life of

a new frog, is deposited in water and hatched by the warmth of spring.

After fertilization the egg or cell divides into two cells, these two

into four, the four into eight, the eight into sixteen, and so on till a

large number of small cells, associated together, is formed (Fig. 10).

These cell phases represent different stages in the life of the growing

frog. It is thus seen that the frog, in its earliest stages, consists of

nothing but many small cells. These cells, through the mysterious powers

of heredity, are going to differentiate as development proceeds into the

various organs (groups of various kinds of cells) that form the tadpole

and finally the frog. For further stages in the development of the egg

(oösperm) towards the tadpole, works upon Comparative Embryology should

be consulted.

At length the tadpole is hatched from the egg, and then soon swims about

in the water (Fig. 17).

The creature that comes from the egg looks nothing whatever like a frog.

It has no limbs whatever, and consists mainly of a bulky head and tail.

This is the tadpole stage in the development of the frog. It can exist

only in water, breathing air therefrom by means of gills. Like the fish,

it has a two-chambered heart. At this stage it has no lungs, and the

gills consist of an external (Fig. 17, a) and an internal pair. The mouth

is small, with only horny toothless jaws, with no tongue. The creature

is herbivorous, living on decaying vegetable matter. The vertebræ of the

spinal column are bi-concave, as in fishes. The tadpole is essentially a

fish, and would be so classed if it did not develop further. An evolving

fish does not go beyond this stage. But the developing frog does go

beyond this stage to a higher one. As its evolution proceeds through the

multiplication and differentiation of the cells that form its body, limbs

begin to bud out, first posteriorly (Fig. 17, b) and then anteriorly

(Fig. 17, c). The lungs now begin to develop, and the external gills

dwindle more and more until they soon disappear, the internal ones

persisting for a while longer. The tongue, at this stage, also makes its

appearance. The creature now can breathe both air and water. This is

the permanent condition of many adult amphibians belonging to a lower

order than the mature frog, such as the _siren_, _menobranchus_, etc.

The siren in developing also passes through the fish stage, but does not

get beyond the siren stage. But the evolving frog does go beyond this

stage, for with the growth of the legs the tail dwindles slowly by its

gradual absorption (Fig. 17, d). The internal gills now disappear through

absorption, and the lungs develop more thoroughly. Great changes take

place in the blood-vascular system, the fish-like, two-chambered heart

evolving into the three-chambered, amphibian heart. In spite of its

dwindling, the tail is still a very conspicuous organ. In this phase of

its development the frog can breathe only air, and must frequently come

to the surface of the water for that purpose, and soon leaves the water

altogether. Now this stage of the creature’s development corresponds to

the permanent adult condition of another order of amphibians, which is

higher than that to which the siren belongs but lower than the order of

the adult frog. This intermediate order has such creatures in it as the

_triton_. The triton in developing passes through the fish and siren

stages, but does not get higher than the triton stage. But the evolving

frog goes even higher than this triton-like condition. Its tail is more

and more absorbed until it finally disappears, and then the young but

perfect frog appears (Fig. 17, e). During this period the teeth develop

and the creature becomes carnivorous, feeding on insects. It is thus seen

that the developing frog passes by small gradations from one class (the

fish class) to an altogether different and higher class (the amphibian

class). When it has evolved to this higher class, it then passes from

the lower order (“siren” order) to a higher one (“triton” order), and

then to the highest order (“frog” order). The bi-concave vertebræ of the

fish-like tadpole have now developed into vertebræ with the cup-and-ball

joints of the higher amphibian. It is the same with all the complex

organs of the adult frog; they evolve from the much simpler structures of

the tadpole.

[Illustration: FIG. 17.—Tadpoles and Frog; a, tadpole with branching

external gills; b, gills absorbed and hind legs have appeared; c, fore

legs have appeared; d, tail shrunk and legs enlarged; e, perfect, young

frog,—tail entirely disappeared. The figures represent some stages in the

life history of the frog.]

This study of the frog’s evolution from the fertilized egg is profoundly

instructive. It reveals to us, through direct observation, that a

creature varies in its form and structure at succeeding intervals of

time. These variations diverge more and more, so that specific, generic,

and even ordinal and class distinctions are revealed as the development

proceeds. Owen, the distinguished comparative anatomist, in speaking of

the transmutation of one species into another in the course of geologic

history, says, though with a hostile purpose in view, that in the

metamorphoses of the amphibians we seem to have such process carried on

before our eyes to its extremest extent. Not merely is one specific

form changed to another of the same genus; not merely is one generic

modification of an order substituted for another, the transmutation is

not even limited by passing from one order (Urodela) to another (Anura);

it affects a transition from class to class. The fish becomes the frog

(amphibian); the aquatic animal changes to the terrestrial one; the

water-breather becomes the air-breather; an insect diet is substituted

for a vegetable one. And these changes, moreover, proceed gradually,

continuously, and without any interruption of active life. Such is the

language of Owen in reference to these remarkable transmutations of the

developing frog.

The development of the frog is a brief recapitulation, an epitome,

through heredity, of the main transmutations of its ancestral forms

in geologic time. It is not true that the embryonic phases in the

development of a higher form always resemble the adult stages of lower

forms. This may or may not be the case; but what always does occur is

that the embryonic phases of a higher form resemble the corresponding

phases of the lower forms. So far as the frog’s development is

concerned, it is very instructive to know that the order of succession

of its embryonic forms undoubtedly parallels the order of succession

of corresponding forms in past geologic ages. Fishes appeared in the

Upper Silurian rocks with amphibian characteristics. In the succeeding

Carboniferous Ages the fishes still continued under new forms; but also

the lowest forms of amphibians, the most fish-like forms, now appeared.

They were somewhat like the sirens, they were perennibranchs. In the next

succeeding rocks, the Permian and Triassic, higher, triton-like forms

appeared. They were caducibranchs. Finally, in the Tertiary rocks, the

highest forms of amphibians are found, such as the frogs.

In order to understand the relation of Ontogeny to Phylogeny, it must be

carefully borne in mind that the simple and lowly organized creatures on

the globe at the first appearance of life were performing the two great

functions that all living creatures perform, viz.: those of nutrition

and reproduction. These functions imply that organisms were reacting to

environment, and, therefore, undergoing modifications and adaptations;

and at the same time the organisms were giving origin to offspring—they

were reproducing their kind through heredity. As these simple organisms

lived through the ages and became more and more complex by modifications

and adaptations to an ever-changing environment, they still evolved

their kind in reproduction. Every new adaptation gained by the parent

was transmitted by heredity, in the course of time, to the offspring;

every form and structure modified in the parent was modified by heredity

in the offspring; and every structure lost by the parent was finally

lost in the offspring. Just in proportion as the parents, through the

ages, became modified, often becoming more complex by the addition of

adaptation to adaptation, retaining some structures of their ancestors

by heredity (through use) and losing others, eventually, through disuse;

so the offspring of these modifying parents became correspondingly

modified, and acquired by heredity the modified structures and habits

of the parents, while losing other structures in time that the parents

had lost. Just as complex _organisms_ of later ages have been evolved

from the simpler organisms of earlier ages by the addition of adaptation

to adaptation, in an orderly sequence (Phylogeny); so, therefore, the

complex _offspring_, while growing, unfold these inherited adaptations

in the order of their acquisition. This last process is called Ontogeny

or Embryology. Ontogeny is undoubtedly an illustration of the results

of Natural Selection’s activity; for, during the phylogeny of the frog

throughout the incalculable ages of the past its ancestors undoubtedly

assumed innumerable forms and structures which were adaptations to

the times and surroundings. But with the advancing time and changing

environment, some of the old forms and structures continued useful and

were retained, while others became useless and were eliminated by Natural

Selection. In addition to the old useful structures that were retained

changing environment often modified some of the retained structures and

added still other adaptations to these. And so on, throughout the ages,

in building up a frog, through geologic embryos, geologic “infants,”

geologic “children,” and finally geologic adult frogs, Natural Selection

has retained during ontogeny many useful structures in the order of

their first appearance, and eliminated innumerable others that became

useless. The ontogeny of the frog, which has been built up by its

phylogeny, reveals the useful structures that have been retained, and in

the order of their appearance; often showing structures that have been

lost in the parent, but are not yet quite lost in the embryo, while it

fails to show innumerable useless structures that have been lost in the

past. This is the reason why we say that the ontogeny of a frog is a

brief outline recapitulation of the main points in the phylogeny of the

frog, with even some main points occasionally omitted altogether. The

geologic ancestors of the frog were the scaffoldings by which it climbed

from simple creatures up to its present complex organization; just as

the embryological phases at present are the scaffoldings by which a

simple, unicellular, fertilized ovum climbs up through heredity to the

huge complexity of the multicellular adult frog. What is true of the

development of the frog, ontogenetically and phylogenetically, is also

true of all living creatures, and is therefore true of =man=.

Man, in his individual development, commences life as a small,

microscopic cell—the fertilized ovum—which is only one-fifth of a

millimeter in size. His first stage resembles an encysted protozoan

animal. As cell-multiplication proceeds he soon gets into the morula

stage, which resembles a colony of undifferentiated protozoans. He soon

evolves into a stage which may be compared to a colony of protozoans some

of the members of which have undergone differentiation. Then comes the

gastrula stage, which is distinctly suggestive of a low metazoan, and

in which the developing germ assumes fundamental anatomical qualities

such as characterize lowly animals like polyps. Then, by gradual

transmutations, the vertebrate characteristics appear; but it could not

be said at this stage of development, if one did not know, whether one

is observing a fish, an amphibian, a reptile, or a mammal. Finally, the

developing man passes through his fish and reptile phases and reaches the

mammal stage. But as yet it cannot be said to which order the animal

belongs. The evolution of the individual continuing, he finally assumes

those anatomical characteristics that stamp him as belonging to the order

of man.

The theory of evolution, then, teaches that this development of man in

the course of a few short months, like the development of the frog, is a

very condensed and abbreviated epitome of the evolution of mankind from

primitive protozoans during the incalculable ages of the past.

Drummond has prettily written that “the developing human embryo is

like a subtle phantasmagoria, a living theater in which a weird

transformation scene is being enacted and in which countless strange and

uncouth characters take part. Some of these characters are well known

to science, some are strangers. As the embryo unfolds, one by one these

animal-actors come upon the stage, file past in phantom-like procession,

throw off their drapery, and dissolve away into something else. Yet,

as they vanish, each leaves behind a vital portion of itself, some

original and characteristic memorial, something itself has made or won,

that perhaps it alone could make or win,—a bone, a muscle, a ganglion,

or a tooth,—to be the inheritance of the race. And it is only after

nearly all have played their part and dedicated their gift that a human

form, mysteriously compounded of all that has gone before, begins to be

discerned as the resultant.”

As has been stated in the introductory part of this book, if all the

animals that have ever lived on the globe should be represented by a tree

those existing on the earth to-day would be indicated by the topmost

twigs and leaves, while the extinct forms would be represented by the

trunk and main branches. Just as the leaves, twigs, branches, and trunk

of the tree have a common origin, viz., the seed that developed into the

tree, so all the different species of animals of the present and the past

are the trunk, branches, twigs, and leaves of the “tree of life,” and

have had a common origin from a primitive protozoan cell (see Diagram of

Development, Fig. 18). Therefore all creatures, living and past, have a

more or less blood relationship.

The Diagram of Development will indicate in a very general way the

possible track taken by a man as he evolved,—grew higher and higher as

the central, straight trunk of the expanding tree of life,—during the

geologic ages; and finally appeared as the inflorescence of the topmost

branch of this central trunk. It is seen from this scheme that the

tree of life commenced in a primitive cell. Without entering into any

discussion of the various theories of evolution and epigenesis, we may

say that the primitive protozoan contained potentially all the animal

forms (each being a cell or group of cells) that have existed on the

globe, just as the fertilized egg contains potentially all the tissues

and organs (groups of cells) of the adult man.

As the tree of animal life unfolded and expanded—like a germinating

seed—from the primitive protozoan, certain of the descendants evolved

along the straight and central branch, through the primitive colonial

protozoans, on through primitive vermes, and still on through primitive

fishes (elasmobranchii), amphibians, reptiles, and on through primitive

ornithodelphia (monotremes), and didelphia (marsupials) to a primitive

order of monodelphia, viz.: primitive primates. The evolution of man

continued through primitive anthropoidea to primitive anthropopithecus.

At this point we meet with the common ancestors of the _higher anthropoid

apes_ (chimpanzee and gorilla) and _man_.

[Illustration: FIG. 18.—Diagram of Development: Portion of the “Tree of

Life,” showing approximately the relative places of the great groups of

animals. The _Central Trunk_ and _Primary Branches_ represent _Primitive_

(geologic) forms; the _Terminal Twigs_ represent _Modern_ forms.]

At each stage of the evolution some of the descendants of the animals of

this stage diverged obliquely, modifying the characters they possessed at

this stage in a direction that varied more and more from those characters

that led on to man. So that all along the central trunk of the tree of

animal life collateral branches were given off. The collateral branches

given off at each upward stage of evolution represent animals higher in

the scale than those that departed from the central trunk lower down.

To illustrate what has occurred at each stage in the evolution of man,

pause for a moment to consider that phase of progress represented by the

primitive reptilia. If we study the anatomy of the specialized reptiles,

birds, and monotremes of the present, we will find that they all have

many characters in common. These characters are _reptilian_. Each class

has its own distinctive specialized peculiarities in addition to its

common reptilian characters. The study of the fossils of the rocks shows

that in the Jurassic and Cretaceous ages animals existed that were

undoubtedly reptiles, but had also very distinct bird characters; also

reptiles existed that had distinct monotreme characters. These reptiles

came from those of earlier times that were still more generalized. As the

ages passed, some of the generalized reptiles (primitive reptiles) lost

more and more the reptilian features and gradually assumed more and more

distinct bird characters, until finally the highly specialized modern

birds (“glorified reptiles”) were evolved as a branch from primitive

reptiles. The specialized reptiles of modern times likewise came from

the primitive reptiles. In like manner those primitive reptiles that had

mammalian (monotreme) characters, by getting into a suitable environment,

gradually lost more and more their reptilian characters and assumed

with increasing accentuation the characters of primitive monotremes—the

lowest of the mammalian class. But observe particularly that the earliest

introduced monotremes were not the specialized monotremes on the globe

to-day, but generalized, primitive monotremes. These gave origin to the

specialized modern monotremes, and also to the generalized primitive

marsupials. The evolution of man continued through the primitive

marsupials to primitive anthropoidea.

Here we meet with the common, generalized ancestors of man and the

monkeys. These creatures contained, potentially at least, _anthropoid_

as well as _pithecoid_ characters. From them were derived the primitive

New World monkeys (primitive Platyrrhines) and the primitive Old

World monkeys (primitive Catarrhines). Some of the descendants of the

primitive Old World monkeys, migrating into an environment which favored

particularly the pithecoid characters, eventually developed into the

tailed monkeys of the Old World (Cercopithecidæ). Others of their

descendants, migrating into a different environment, found conditions

that favored the anthropoid characters especially, and by greater

and greater use of these, with the diminished use of the pithecoid

ones, the characters of the anthropoid apes (primitive Simiidæ and

primitive Simiinæ) became clearer and clearer until, in time, primitive

anthropopithecus appeared,—a tailless anthropoid ape of the Old World.

More than likely this anthropoid ape bore a close resemblance, as Dr.

Theodore Gill long since taught, to the modern chimpanzee. If there were

any differences they could scarcely have been of even a generic value.

This primitive chimpanzee was undoubtedly a quadrupedal, quadrumanous

creature leading an arboreal life. His descendants specialized along

two distinct but closely related lines. Those that continued to live in

trees specialized along the oblique path that led finally to the gorilla

on the one hand and the chimpanzee on the other. Those descendants that

abandoned the trees and lived on the ground used the feet more and more

for purposes of locomotion and less for grasping; while they employed

with increasing frequency the hands for grasping exclusively. Associated

with these adaptations were many other correlated adaptations, such as

the upright posture, an enlarging brain, a change in the character of the

face and of the dentition, etc. As man evolved further and further along

the central trunk of the tree of life, he discarded, through disuse, many

of the characters that are peculiar to the anthropoid apes; and assumed

with increasing emphasis, through use, many of the characters that are

distinctive of man. He passed through the phases of pithecoid man and

pre-palæolithic (primitive) man, until eventually, in palæolithic man,

the visage of humanity is clear and unmistakable.

It is extremely interesting to attempt to form some rough picture of

primitive man. It may help us to do so if we recall what Darwin has said

about the Fuegians, who are among the lowest of savages. He has written

that they are men whose very signs and expressions are less intelligible

to us than those of the domesticated animals—men who do not possess the

instinct of those animals, nor yet appear to boast of human reason, or at

least of arts consequent on that reason.

The Fuegians are much nearer to the ape than to a Shakespeare or Sir

Isaac Newton. In the words of Clodd, primitive man was doubtless much

lower than the lowest Fuegians. “He was a powerful, cunning biped, with

keen sense organs (always sharper, in virtue of constant exercise, in

the savage than in the civilized man, who supplements them by science),

strong instincts, uncontrolled and fitful emotions, small faculty of

wonder, and nascent reasoning power; unable to forecast to-morrow or to

comprehend yesterday, living from hand to mouth on the wild products of

nature, clothed in skin or bark, or daubed with clay, and finding shelter

in trees and caves; ignorant of the simplest arts, save to chip a stone

missile, and perhaps to produce fire; strong in his need of life and

vague sense of right to it and to what he could get, but slowly impelled

by common perils and passions to form ties, loose and haphazard at the

outset, with his kind, the power of combination with them depending on

sounds, signs, and gestures.”

Through the theory of evolution it can readily be understood why the

anatomical characters of the anthropoid apes and of man are so very

closely alike. They have a common origin, and are blood relations—the

one group of animals having specialized from common ancestors in one

direction (obliquely), and the other group having specialized in another

(straight) direction. (Vid. Diagram of Development.)

Man, in his individual development from a fertilized ovum, comes from a

source infinitely lower than the ape. Why, therefore, should he feel such

reluctance to believe that he has passed, during geologic ages, through

the phase of generalized simian ancestors? Is there not much more of hope

in the knowledge that he has risen higher and higher through the æons of

the past than in the belief that he was created an innocent and noble

character and then fell to utter wretchedness through great temptation?

The motto of evolution is Excelsior. For it shows that the human race,

through all the incalculable ages of the past, has risen to higher and

higher levels,—to nobler and nobler phases of being. His progress in

the almost infinite past suggests the hope that he will mount higher

and higher towards perfection during the limitless future. Not only may

we hope that there will be boundless improvement of the human race, but

boundless evolution of each individual human being as well. Evolution’s

motto for each individual may also be Excelsior. And, therefore,

may we say with some assurance of hope that love, while kissing the

pathetic lips of death, need not entertain in vain the splendid hope of

immortality. For if there be no immortality of personal consciousness,

then the evolution of the cosmos, of man, of the highest mind in man,

have no intelligible meaning for us; they are unfathomable enigmas—idiot

stories without meaning.

Man, in specializing along certain lines since separating from the

ancestral simian stock, has displayed more and more that structure of

his skeleton and of the soft parts molded upon it that is best adapted

to the needs of the mind resident within him. His bones are not merely

the jointed framework of an animal, but a framework adapted to that erect

attitude which so befits his intellectual nature. His feet are not the

climbing and grasping feet of the ape, but organs for giving firmness

to the tread and dignity to the bearing of a creature capable of high

thought. The arms and hands are not for strength alone, for these members

are much stronger in many a brute; but they also give greater expression

and power to the thoughts that come from within. The hands possess

such molding of fingers, thumbs, and palms, such delicacy for tactile

impressions, and such capacity for nice adjustments, that they are not

alone used for feeding the mouth and fighting antagonists; but they also

contribute pre-eminently to the desires of a large mind, and are the

efficient servants of its promptings. As Dana well says, “The face, with

its expressive features, is formed so as to respond not solely to the

emotions of pleasure and pain, but to shades of sentiment and interacting

sympathies the most varied, high as heaven and low as earth,—ay, lower,

in debased human nature; the whole being, body, limbs, and head, with

eyes looking, not towards the earth, but beyond an infinite horizon, is a

majestic expression of the divine feature in man and of the infinitude of

his aspirations.”

It ought now to be understood that not only is the present environment