THE GEOLOGICAL EVIDENCES OF EVOLUTION
What we may expect--The number of known species of extinct
animals--Causes of the imperfection of the geological
record--Geological evidences of
evolution--Shells--Crocodiles--The rhinoceros tribe--The
pedigree of the horse tribe--Development of deer's horns--Brain
development--Local relations of fossil and living animals--Cause
of extinction of large animals--Indications of general progress
in plants and animals--The progressive development of
plants--Possible cause of sudden late appearance of
exogens--Geological distribution of insects--Geological
succession of vertebrata--Concluding remarks.
The theory of evolution in the organic world necessarily implies that
the forms of animals and plants have, broadly speaking, progressed from
a more generalised to a more specialised structure, and from simpler to
more complex forms. We know, however, that this progression has been by
no means regular, but has been accompanied by repeated degradation and
degeneration; while extinction on an enormous scale has again and again
stopped all progress in certain directions, and has often compelled a
fresh start in development from some comparatively low and imperfect
type.
The enormous extension of geological research in recent times has made
us acquainted with a vast number of extinct organisms, so vast that in
some important groups--such as the mollusca--the fossil are more
numerous than the living species; while in the mammalia they are not
much less numerous, the preponderance of living species being chiefly in
the smaller and in the arboreal forms which have not been so well
preserved as the members of the larger groups. With such a wealth of
material to illustrate the successive stages through which animals have
passed, it will naturally be expected that we should find important
evidence of evolution. We should hope to learn the steps by which some
isolated forms have been connected with their nearest allies, and in
many cases to have the gaps filled up which now separate genus from
genus, or species from species. In some cases these expectations are
fulfilled, but in many other cases we seek in vain for evidence of the
kind we desire; and this absence of evidence with such an apparent
wealth of material is held by many persons to throw doubt on the theory
of evolution itself. They urge, with much appearance of reason, that all
the arguments we have hitherto adduced fall short of demonstration, and
that the crucial test consists in being able to show, in a great number
of cases, those connecting links which we say must have existed. Many of
the gaps that still remain are so vast that it seems incredible to these
writers that they could ever have been filled up by a close succession
of species, since these must have spread over so many ages, and have
existed in such numbers, that it seems impossible to account for their
total absence from deposits in which great numbers of species belonging
to other groups are preserved and have been discovered. In order to
appreciate the force, or weakness, of these objections, we must inquire
into the character and completeness of that record of the past life of
the earth which geology has unfolded, and ascertain the nature and
amount of the evidence which, under actual conditions, we may expect to
find.
_The Number of known Species of Extinct Animals._
When we state that the known fossil mollusca are considerably more
numerous than those which now live on the earth, it appears at first
sight that our knowledge is very complete, but this is far from being
the case. The species have been continually changing throughout
geological time, and at each period have probably been as numerous as
they are now. If we divide the fossiliferous strata into twelve great
divisions--the Pliocene, Miocene, Eocene, Cretaceous, Oolite, Lias,
Trias, Permian, Carboniferous, Devonian, Silurian, and Cambrian,--we
find not only that each has a very distinct and characteristic molluscan
fauna, but that the different subdivisions often present a widely
different series of species; so that although a certain number of
species are common to two or more of the great divisions, the totality
of the species that have lived upon the earth must be very much more
than twelve times--perhaps even thirty or forty times--the number now
living. In like manner, although the species of fossil mammals now
recognised by more or less fragmentary fossil remains may not be much
less numerous than the living species, yet the duration of existence of
these was comparatively so short that they were almost completely
changed, perhaps six or seven times, during the Tertiary period; and
this is certainly only a fragment of the geological time during which
mammalia existed on the globe.
There is also reason to believe that the higher animals were much more
abundant in species during past geological epochs than now, owing to the
greater equability of the climate which rendered even the arctic regions
as habitable as the temperate zones are in our time.
The same equable climate would probably cause a more uniform
distribution of moisture, and render what are now desert regions capable
of supporting abundance of animal life. This is indicated by the number
and variety of the species of large animals that have been found fossil
in very limited areas which they evidently inhabited at one period. M.
Albert Gaudry found, in the deposits of a mountain stream at Pikermi in
Greece, an abundance of large mammalia such as are nowhere to be found
living together at the present time. Among them were two species of
Mastodon, two different rhinoceroses, a gigantic wild boar, a camel and
a giraffe larger than those now living, several monkeys, carnivora
ranging from martens and civets to lions and hyaenas of the largest
size, numerous antelopes of at least five distinct genera, and besides
these many forms altogether extinct. Such were the great herds of
Hipparion, an ancestral form of horse; the Helladotherium, a huge animal
bigger than the giraffe; the Ancylotherium, one of the Edentata; the
huge Dinotherium; the Aceratherium, allied to the rhinoceros; and the
monstrous Chalicotherium, allied to the swine and ruminants, but as
large as a rhinoceros; and to prey upon these, the great Machairodus or
sabre-toothed tiger. And all these remains were found in a space 300
paces long by 60 paces broad, many of the species existing in enormous
quantities.
The Pikermi fossils belong to the Upper Miocene formation, but an
equally rich deposit of Upper Eocene age has been discovered in
South-Western France at Quercy, where M. Filhol has determined the
presence of no less than forty-two species of beasts of prey alone.
Equally remarkable are the various discoveries of mammalian fossils in
North America, especially in the old lake bottoms now forming what are
called the "bad lands" of Dakota and Nebraska, belonging to the Miocene
period. Here are found an enormous assemblage of remains, often perfect
skeletons, of herbivora and carnivora, as varied and interesting as
those from the localities already referred to in Europe; but altogether
distinct, and far exceeding, in number and variety of species of the
larger animals, the whole existing fauna of North America. Very similar
phenomena occur in South America and in Australia, leading us to the
conclusion that the earth at the present time is impoverished as regards
the larger animals, and that at each successive period of Tertiary time,
at all events, it contained a far greater number of species than now
inhabit it. The very richness and abundance of the remains which we find
in limited areas, serve to convince us how imperfect and fragmentary
must be our knowledge of the earth's fauna at any one past epoch; since
we cannot believe that all, or nearly all, of the animals which
inhabited any district were entombed in a single lake, or overwhelmed by
the floods of a single river.
But the spots where such rich deposits occur are exceedingly few and far
between when compared with the vast areas of continental land, and we
have every reason to believe that in past ages, as now, numbers of
curious species were rare or local, the commoner and more abundant
species giving a very imperfect idea of the existing series of animal
forms. Yet more important, as showing the imperfection of our knowledge,
is the enormous lapse of time between the several formations in which we
find organic remains in any abundance, so vast that in many cases we
find ourselves almost in a new world, all the species and most of the
genera of the higher animals having undergone a complete change.
_Causes of the Imperfection of the Geological Record._
These facts are quite in accordance with the conclusions of geologists
as to the necessary imperfection of the geological record, since it
requires the concurrence of a number of favourable conditions to
preserve any adequate representation of the life of a given epoch. In
the first place, the animals to be preserved must not die a natural
death by disease, or old age, or by being the prey of other animals, but
must be destroyed by some accident which shall lead to their being
embedded in the soil. They must be either carried away by floods, sink
into bogs or quicksands, or be enveloped in the mud or ashes of a
volcanic eruption; and when thus embedded they must remain undisturbed
amid all the future changes of the earth's surface.
But the chances against this are enormous, because denudation is always
going on, and the rocks we now find at the earth's surface are only a
small fragment of those which were originally laid down. The
alternations of marine and freshwater deposits, and the frequent
unconformability of strata with those which overlie them, tell us
plainly of repeated elevations and depressions of the surface, and of
denudation on an enormous scale. Almost every mountain range, with its
peaks, ridges, and valleys, is but the remnant of some vast plateau
eaten away by sub-aerial agencies; every range of sea-cliffs tell us of
long slopes of land destroyed by the waves; while almost all the older
rocks which now form the surface of the earth have been once covered
with newer deposits which have long since disappeared. Nowhere are the
evidences of this denudation more apparent than in North and South
America, where granitic or metamorphic rocks cover an area hardly less
than that of all Europe. The same rocks are largely developed in Central
Africa and Eastern Asia; while, besides those portions that appear
exposed on the surface, areas of unknown extent are buried under strata
which rest on them uncomformably, and could not, therefore, constitute
the original capping under which the whole of these rocks must once have
been deeply buried; because granite can only be formed, and metamorphism
can only go on, deep down in the crust of the earth. What an
overwhelming idea does this give us of the destruction of whole piles
of rock, miles in thickness and covering areas comparable with those of
continents; and how great must have been the loss of the innumerable
fossil forms which those rocks contained! In view of such destruction we
are forced to conclude that our palaeontological collections, rich
though they may appear, are really but small and random samples, giving
no adequate idea of the mighty series of organism which have lived upon
the earth.[183]
Admitting, however, the extreme imperfection of the geological record as
a whole, it may be urged that certain limited portions of it are fairly
complete--as, for example, the various Miocene deposits of India,
Europe, and North America,--and that in these we ought to find many
examples of species and genera linked together by intermediate forms. It
may be replied that in several cases this really occurs; and the reason
why it does not occur more often is, that the theory of evolution
requires that distinct genera should be linked together, not by a direct
passage, but by the descent of both from a common ancestor, which may
have lived in some much earlier age the record of which is either
wanting or very incomplete. An illustration given by Mr. Darwin will
make this more clear to those who have not studied the subject. The
fantail and pouter pigeons are two very distinct and unlike breeds,
which we yet know to have been both derived from the common wild
rock-pigeon. Now, if we had every variety of living pigeon before us, or
even all those which have lived during the present century, we should
find no intermediate types between these two--none combining in any
degree the characters of the pouter with that of the fantail. Neither
should we ever find such an intermediate form, even had there been
preserved a specimen of every breed of pigeon since the ancestral
rock-pigeon was first tamed by man--a period of probably several
thousand years. We thus see that a complete passage from one very
distinct species to another could not be expected even had we a complete
record of the life of any one period. What we require is a complete
record of all the species that have existed since the two forms began
to diverge from their common ancestor, and this the known imperfection
of the record renders it almost impossible that we should ever attain.
All that we have a right to expect is, that, as we multiply the fossil
forms in any group, the gaps that at first existed in that group shall
become less wide and less numerous; and also that, in some cases, a
tolerably direct series shall be found, by which the more specialised
forms of the present day shall be connected with more generalised
ancestral types. We might also expect that when a country is now
characterised by special groups of animals, the fossil forms that
immediately preceded them shall, for the most part, belong to the same
groups; and further, that, comparing the more ancient with the more
modern types, we should find indications of progression, the earlier
forms being, on the whole, lower in organisation, and less specialised
in structure than the later. Now evidence of evolution of these varied
kinds is what we do find, and almost every fresh discovery adds to their
number and cogency. In order, therefore, to show that the testimony
given by geology is entirely in favour of the theory of descent with
modification, some of the more striking of the facts will now be given.
_Geological Evidences of Evolution._
In an article in _Nature_ (vol. xiv. p. 275), Professor Judd calls
attention to some recent discoveries in the Hungarian plains, of fossil
lacustrine shells, and their careful study by Dr. Neumayr and M. Paul of
the Austrian Geological Survey. The beds in which they occur have
accumulated to the thickness of 2000 feet, containing throughout
abundance of fossils, and divisible into eight zones, each of which
exhibits a well-marked and characteristic fauna. Professor Judd then
describes the bearing of these discoveries as follows--
"The group of shells which affords the most interesting evidence
of the origin of new forms through descent with modification is
that of the genus Vivipara or Paludina, which occurs in
prodigious abundance throughout the whole series of freshwater
strata. We shall not, of course, attempt in this place to enter
into any details concerning the forty distinct _forms_ of this
genus (Dr. Neumayr very properly hesitates to call them all
_species_), which are named and described in this monograph,
and between which, as the authors show, so many connecting
links, clearly illustrating the derivation of the newer from the
older types, have been detected. On the minds of those who
carefully examine the admirably engraved figures given in the
plates accompanying this valuable memoir, or still better, the
very large series of specimens from among which the subjects of
these figures are selected, and which are now in the museum of
the Reichsanstalt of Vienna, but little doubt will, we suspect,
remain that the authors have fully made out their case, and have
demonstrated that, beyond all controversy, the series with
highly complicated ornamentation were variously derived by
descent--the lines of which are in most cases perfectly clear
and obvious--from the simple and unornamented Vivipara
achatinoides of the Congerien-Schichten (the lower division of
the series of strata). It is interesting to notice that a large
portion of these unquestionably derived forms depart so widely
from the type of the genus Vivipara, that they have been
separated on so high an authority as that of Sandberger, as a
new genus, under the name of Tulotoma. And hence we are led to
the conclusion that a vast number of forms, certainly exhibiting
specific distinctions, and according to some naturalists,
differences even entitled to be regarded of generic value, have
all a common ancestry."
It is, as Professor Judd remarks, owing to the exceptionally favourable
circumstances of a long-continued and unbroken series of deposits being
formed under physical conditions either identical or very slowly
changing, that we owe so complete a record of the process of organic
change. Usually, some disturbing elements, such as a sudden change of
physical conditions, or the immigration of new sets of forms from other
areas and the consequent retreat or partial extinction of the older
fauna, interferes with the continuity of organic development, and
produces those puzzling discordances so generally met with in geological
formations of marine origin. While a case of the kind now described
affords evidence of the origin of species complete and conclusive,
though on a necessarily very limited scale, the very rarity of the
conditions which are essential to such completeness serves to explain
why it is that in most cases the direct evidence of evolution is not to
be obtained.
Another illustration of the filling up of gaps between existing groups
is afforded by Professor Huxley's researches on fossil crocodiles. The
gap between the existing crocodiles and the lizards is very wide, but as
we go back in geological time we meet with fossil forms which are to
some extent intermediate and form a connected series. The three living
genera--Crocodilus, Alligator, and Gavialis--are found in the Eocene
formation, and allied forms of another genus, Holops, in the Chalk. From
the Chalk backward to the Lias another group of genera occurs, having
anatomical characteristics intermediate between the living crocodiles
and the most ancient forms. These, forming two genera Belodon and
Stagonolepis, are found in a still older formation, the Trias. They have
characters resembling some lizards, especially the remarkable Hatteria
of New Zealand, and have also some resemblances to the
Dinosaurians--reptiles which in some respects approach birds.
Considering how comparatively few are the remains of this group of
animals, the evidence which it affords of progressive development is
remarkably clear.[184]
Among the higher animals the rhinoceros, the horse, and the deer afford
good evidence of advance in organisation and of the filling up of the
gaps which separate the living forms from their nearest allies. The
earliest ancestral forms of the rhinoceroses occur in the Middle Eocene
of the United States, and were to some extent intermediate between the
rhinoceros and tapir families, having like the latter four toes to the
front feet, and three to those behind. These are followed in the Upper
Eocene by the genus Amynodon, in which the skull assumes more distinctly
the rhinocerotic type. Following this in the Lower Miocene we have the
Aceratherium, like the last in its feet, but still more decidedly a
rhinoceros in its general structure. From this there are two diverging
lines--one in the Old World, the other in the New. In the former, to
which the Aceratherium is supposed to have migrated in early Miocene
times, when a mild climate and luxuriant vegetation prevailed far within
the arctic circle, it gave rise to the Ceratorhinus and the various
horned rhinoceroses of late Tertiary times and of those now living. In
America a number of large hornless rhinoceroses were developed--they
are found in the Upper Miocene, Pliocene, and Post-Pliocene
formations--and then became extinct. The true rhinoceroses have three
toes on all the feet.[185]
_The Pedigree of the Horse Tribe._
Yet more remarkable is the evidence afforded by the ancestral forms of
the horse tribe which have been discovered in the American tertiaries.
The family Equidae, comprising the living horse, asses, and zebras,
differ widely from all other mammals in the peculiar structure of the
feet, all of which terminate in a single large toe forming the hoof.
They have forty teeth, the molars being formed of hard and soft material
in crescentic folds, so as to be a powerful agent in grinding up hard
grasses and other vegetable food. The former peculiarities depend upon
modifications of the skeleton, which have been thus described by
Professor Huxley:--
"Let us turn in the first place to the fore-limb. In most
quadrupeds, as in ourselves, the fore-arm contains distinct
bones, called the radius and the ulna. The corresponding region
in the horse seems at first to possess but one bone. Careful
observation, however, enables us to distinguish in this bone a
part which clearly answers to the upper end of the ulna. This is
closely united with the chief mass of the bone which represents
the radius, and runs out into a slender shaft, which may be
traced for some distance downwards upon the back of the radius,
and then in most cases thins out and vanishes. It takes still
more trouble to make sure of what is nevertheless the fact, that
a small part of the lower end of the bone of a horse's fore-arm,
which is only distinct in a very young foal, is really the lower
extremity of the ulna.
"What is commonly called the knee of a horse is its wrist. The
'cannon bone' answers to the middle bone of the five metacarpal
bones which support the palm of the hand in ourselves. The
pastern, coronary, and coffin bones of veterinarians answer to
the joints of our middle fingers, while the hoof is simply a
greatly enlarged and thickened nail. But if what lies below the
horse's 'knee' thus corresponds to the middle finger in
ourselves, what has become of the four other fingers or digits?
We find in the places of the second and fourth digits only two
slender splintlike bones, about two-thirds as long as the cannon
bone, which gradually taper to their lower ends and bear no
finger joints, or, as they are termed, phalanges. Sometimes,
small bony or gristly nodules are to be found at the bases of
these two metacarpal splints, and it is probable that these
represent rudiments of the first and fifth toes. Thus, the part
of the horse's skeleton which corresponds with that of the human
hand, contains one overgrown middle digit, and at least two
imperfect lateral digits; and these answer, respectively, to the
third, the second, and the fourth fingers in man.
"Corresponding modifications are found in the hind limb. In
ourselves, and in most quadrupeds, the leg contains two distinct
bones, a large bone, the tibia, and a smaller and more slender
bone, the fibula. But, in the horse, the fibula seems, at first,
to be reduced to its upper end; a short slender bone united with
the tibia, and ending in a point below, occupying its place.
Examination of the lower end of a young foal's shin-bone,
however, shows a distinct portion of osseous matter which is the
lower end of the fibula; so that the, apparently single, lower
end of the shin-bone is really made up of the coalesced ends of
the tibia and fibula, just as the, apparently single, lower end
of the fore-arm bone is composed of the coalesced radius and
ulna.
"The heel of the horse is the part commonly known as the hock.
The hinder cannon bone answers to the middle metatarsal bone of
the human foot, the pastern, coronary, and coffin bones, to the
middle toe bones; the hind hoof to the nail; as in the forefoot.
And, as in the forefoot, there are merely two splints to
represent the second and the fourth toes. Sometimes a rudiment
of a fifth toe appears to be traceable.
"The teeth of a horse are not less peculiar than its limbs. The
living engine, like all others, must be well stoked if it is to
do its work; and the horse, if it is to make good its wear and
tear, and to exert the enormous amount of force required for its
propulsion, must be well and rapidly fed. To this end, good
cutting instruments and powerful and lasting crushers are
needful. Accordingly, the twelve cutting teeth of a horse are
close-set and concentrated in the forepart of its mouth, like so
many adzes or chisels. The grinders or molars are large, and
have an extremely complicated structure, being composed of a
number of different substances of unequal hardness. The
consequence of this is that they wear away at different rates;
and, hence, the surface of each grinder is always as uneven as
that of a good millstone."[186]
We thus see that the Equidae differ very widely in structure from most
other mammals. Assuming the truth of the theory of evolution, we should
expect to find traces among extinct animals of the steps by which this
great modification has been effected; and we do really find traces of
these steps, imperfectly among European fossils, but far more completely
among those of America.
It is a singular fact that, although no horse inhabited America when
discovered by Europeans, yet abundance of remains of extinct horses have
been found both in North and South America in Post-Tertiary and Upper
Pliocene deposits; and from these an almost continuous series of
modified forms can be traced in the Tertiary formation, till we reach,
at the very base of the series, a primitive form so unlike our perfected
animal, that, had we not the intermediate links, few persons would
believe that the one was the ancestor of the other. The tracing out of
this marvellous history we owe chiefly to Professor Marsh of Yale
College, who has himself discovered no less than thirty species of
fossil Equidae; and we will allow him to tell the story of the
development of the horse from a humble progenitor in his own words.
"The oldest representative of the horse at present known is the
diminutive Eohippus from the Lower Eocene. Several species have
been found, all about the size of a fox. Like most of the early
mammals, these ungulates had forty-four teeth, the molars with
short crowns and quite distinct in form from the premolars. The
ulna and fibula were entire and distinct, and there were four
well-developed toes and a rudiment of another on the forefeet,
and three toes behind. In the structure of the feet and teeth,
the Eohippus unmistakably indicates that the direct ancestral
line to the modern horse has already separated from the other
perissodactyles, or odd-toed ungulates.
"In the next higher division of the Eocene another genus,
Orohippus, makes its appearance, replacing Eohippus, and showing
a greater, though still distant, resemblance to the equine type.
The rudimentary first digit of the forefoot has disappeared, and
the last premolar has gone over to the molar series. Orohippus
was but little larger than Eohippus, and in most other respects
very similar. Several species have been found, but none occur
later than the Upper Eocene.
"Near the base of the Miocene, we find a third closely allied
genus, Mesohippus, which is about as large as a sheep, and one
stage nearer the horse. There are only three toes and a
rudimentary splint on the forefeet, and three toes behind. Two
of the premolar teeth are quite like the molars. The ulna is no
longer distinct or the fibula entire, and other characters show
clearly that the transition is advancing.
"In the Upper Miocene Mesohippus is not found, but in its place
a fourth form, Miohippus, continues the line. This genus is near
the Anchitherium of Europe, but presents several important
differences. The three toes in each foot are more nearly of a
size, and a rudiment of the fifth metacarpal bone is retained.
All the known species of this genus are larger than those of
Mesohippus, and none of them pass above the Miocene formation.
"The genus Protohippus of the Lower Pliocene is yet more equine,
and some of its species equalled the ass in size. There are
still three toes on each foot, but only the middle one,
corresponding to the single toe of the horse, comes to the
ground. This genus resembles most nearly the Hipparion of
Europe.
"In the Pliocene we have the last stage of the series before
reaching the horse, in the genus Pliohippus, which has lost the
small hooflets, and in other respects is very equine. Only in
the Upper Pliocene does the true Equus appear and complete the
genealogy of the horse, which in the Post-Tertiary roamed over
the whole of North and South America, and soon after became
extinct. This occurred long before the discovery of the
continent by Europeans, and no satisfactory reason for the
extinction has yet been given. Besides the characters I have
mentioned, there are many others in the skeleton, skull, teeth,
and brain of the forty or more intermediate species, which show
that the transition from the Eocene Eohippus to the modern Equus
has taken place in the order indicated"[187] (see Fig. 33).
[Illustration: FIG. 33.--Geological development of the horse tribe
(Eohippus since discovered).]
Well may Professor Huxley say that this is demonstrative evidence of
evolution; the doctrine resting upon exactly as secure a foundation as
did the Copernican theory of the motions of the heavenly bodies at the
time of its promulgation. Both have the same basis--the coincidence of
the observed facts with the theoretical requirements.
_Development of Deer's Horns._
Another clear and unmistakable proof of evolution is afforded by one of
the highest and latest developed tribes of mammals--the true deer. These
differ from all other ruminants in possessing solid deciduous horns
which are always more or less branched. They first appear in the Middle
Miocene formation, and continue down to our time; and their development
has been carefully traced by Professor Boyd Dawkins, who thus summarises
his results:--
"In the middle stage of the Miocene the cervine antler consists
merely of a simple forked crown (as in Cervus dicroceros), which
increases in size in the Upper Miocene, although it still
remains small and erect, like that of the roe. In Cervus
Matheroni it measures 11·4 inches, and throws off not more than
four tines, all small. The deer living in Auvergne in the
succeeding or Pliocene age, present us with another stage in the
history of antler development. There, for the first time, we see
antlers of the Axis and Rusa type, larger and longer, and more
branching than any antlers were before, and possessing three or
more well-developed tines. Deer of this type abounded in
Pliocene Europe. They belong to the Oriental division of the
Cervidae, and their presence in Europe confirms the evidence of
the flora, brought forward by the Comte de Saporta, that the
Pliocene climate was warm. They have probably disappeared from
Europe in consequence of the lowering of the temperature in the
Pleistocene age, while their descendants have found a congenial
home in the warmer regions of Eastern Asia.
"In the latest stage of the Pliocene--the Upper Pliocene of the
Val d'Arno--the Cervus dicranios of Nesti presents us with
antlers much smaller than those of the Irish elk, but very
complicated in their branching. This animal survived into the
succeeding age, and is found in the pre-glacial forest bed of
Norfolk, being described by Dr. Falconer under the name of
Sedgwick's deer. The Irish elk, moose, stag, reindeer, and
fallow deer appear in Europe in the Pleistocene age, all with
highly complicated antlers in the adult, and the first
possessing the largest antlers yet known. Of these the Irish elk
disappeared in the Prehistoric age, after having lived in
countless herds in Ireland, while the rest have lived on into
our own times in Euro-Asia, and, with the exception of the last,
also in North America.
"From this survey it is obvious that the cervine antlers have
increased in size and complexity from the Mid-Miocene to the
Pleistocene age, and that their successive changes are analogous
to those which are observed in the development of antlers in the
living deer, which begin with a simple point, and increase in
number of tines till their limit of growth be reached. In other
words, the development of antlers indicated at successive and
widely-separated pages of the geological record is the same as
that observed in the history of a single living species. It is
also obvious that the progressive diminution of size and
complexity in the antlers, from the present time back into the
early Tertiary age, shows that we are approaching the zero of
antler development in the Mid-Miocene. No trace of any
antler-bearing ruminant has been met with in the lower Miocenes,
either of Europe or the United States."[188]
_Progressive Brain-Development._
The three illustrations now given sufficiently prove that, whenever the
geological record approaches to completeness, we have evidence of the
progressive change of species in definite directions, and from less
developed to more developed types--exactly such a change as we may
expect to find if the evolution theory be the true one. Many other
illustrations of a similar change could be given, but the animal groups
in which they occur being less familiar, the details would be less
interesting, and perhaps hardly intelligible. There is, however, one
very remarkable proof of development that must be briefly noticed--that
afforded by the steady increase in the size of the brain. This may be
best stated in the words of Professor Marsh:--
"The real progress of mammalian life in America, from the
beginning of the Tertiary to the present, is well illustrated by
the brain-growth, in which we have the key to many other
changes. The earliest known Tertiary mammals all had very small
brains, and in some forms this organ was proportionally less
than in certain reptiles. There was a gradual increase in the
size of the brain during this period, and it is interesting to
find that this growth was mainly confined to the cerebral
hemispheres, or higher portion of the brain. In most groups of
mammals the brain has gradually become more convoluted, and thus
increased in quality as well as quantity. In some also the
cerebellum and olfactory lobes, the lower parts of the brain,
have even diminished in size. In the long struggle for existence
during Tertiary time the big brains won, then as now; and the
increasing power thus gained rendered useless many structures
inherited from primitive ancestors, but no longer adapted to new
conditions."
This remarkable proof of development in the organ of the mental
faculties, forms a fitting climax to the evidence already adduced of the
progressive evolution of the general structure of the body, as
illustrated by the bony skeleton. We now pass on to another class of
facts equally suggestive of evolution.
_The Local Relations of Fossil and Living Animals._
If all existing animals have been produced from ancestral forms--mostly
extinct--under the law of variation and natural selection, we may expect
to find in most cases a close relation between the living forms of each
country and those which inhabited it in the immediately preceding epoch.
But if species have originated in some quite different way, either by
any kind of special creation, or by sudden advances of organisation in
the offspring of preceding types, such close relationship would not be
found; and facts of this kind become, therefore, to some extent a test
of evolution under natural selection or some other law of gradual
change. Of course the relationship will not appear when extensive
migration has occurred, by which the inhabitants of one region have been
able to take possession of another region, and destroy or drive out its
original inhabitants, as has sometimes happened. But such cases are
comparatively rare, except where great changes of climate are known to
have occurred; and we usually do find a remarkable continuity between
the existing fauna and flora of a country and those of the immediately
preceding age. A few of the more remarkable of these cases will now be
briefly noticed.
The mammalian fauna of Australia consists, as is well known, wholly of
the lowest forms--the Marsupials and Monotremata--except only a few
species of mice. This is accounted for by the complete isolation of the
country from the Asiatic continent during the whole period of the
development of the higher animals. At some earlier epoch the ancestral
marsupials, which abounded both in Europe and North America in the
middle of the Secondary period, entered the country, and have since
remained there, free from the competition of higher forms, and have
undergone a special development in accordance with the peculiar
conditions of a limited area. While in the large continents higher forms
of mammalia have been developed, which have almost or wholly
exterminated the less perfect marsupials, in Australia these latter have
become modified into such varied forms as the leaping kangaroos, the
burrowing wombats, the arboreal phalangers, the insectivorous
bandicoots, and the carnivorous Dasyuridae or native cats, culminating
in the Thylacinus or "tiger-wolf" of Tasmania--animals as unlike each
other as our sheep, rabbits, squirrels, and dogs, but all retaining the
characteristic features of the marsupial type.
Now in the caves and late Tertiary or Post-Tertiary deposits of
Australia the remains of many extinct mammalia have been found, but all
are marsupials. There are many kangaroos, some larger than any living
species, and others more allied to the tree-kangaroos of New Guinea; a
large wombat as large as a tapir; the Diprotodon, a thick-limbed
kangaroo the size of a rhinoceros or small elephant; and a quite
different animal, the Nototherium, nearly as large. The carnivorous
Thylacinus of Tasmania is also found fossil; and a huge phalanger,
Thylacoleo, the size of a lion, believed by Professor Owen and by
Professor Oscar Schmidt to have been equally carnivorous and
destructive.[189] Besides these, there are many other species more
resembling the living forms both in size and structure, of which they
may be, in some cases, the direct ancestors. Two species of extinct
Echidna, belonging to the very low Monotremata, have also been found in
New South Wales.
Next to Australia, South America possesses the most remarkable
assemblage of peculiar mammals, in its numerous Edentata--the sloths,
ant-eaters, and armadillos; its rodents, such as the cavies and
chinchillas; its marsupial opossums, and its quadrumana of the family
Cebidae. Remains of extinct species of all these have been found in the
caves of Brazil, of Post-Pliocene age; while in the earlier Pliocene
deposits of the pampas many distinct genera of these groups have been
found, some of gigantic size and extraordinary form. There are
armadillos of many types, some being as large as elephants; gigantic
sloths of the genera Megatherium, Megalonyx, Mylodon, Lestodon, and many
others; rodents belonging to the American families Cavidae and
Chinchillidae; and ungulates allied to the llama; besides many other
extinct forms of intermediate types or of uncertain affinities.[190] The
extinct Moas of New Zealand--huge wingless birds allied to the living
Apteryx--illustrate the same general law.
The examples now quoted, besides illustrating and enforcing the general
fact of evolution, throw some light on the usual character of the
modification and progression of animal forms. In the cases where the
geological record is tolerably complete, we find a continuous
development of some kind--either in complexity of ornamentation, as in
the fossil Paludinas of the Hungarian lake-basins; in size and in the
specialisation of the feet and teeth, as in the American fossil horses;
or in the increased development of the branching horns, as in the true
deer. In each of these cases specialisation and adaptation to the
conditions of the environment appear to have reached their limits, and
any change of these conditions, especially if it be at all rapid or
accompanied by the competition of less developed but more adaptable
forms, is liable to cause the extinction of the most highly developed
groups. Such we know was the case with the horse tribe in America, which
totally disappeared in that continent at an epoch so recent that we
cannot be sure that the disappearance was not witnessed, perhaps caused,
by man; while even in the Eastern hemisphere it is the smaller
species--the asses and the zebras--that have persisted, while the larger
and more highly developed true horses have almost, if not quite,
disappeared in a state of nature. So we find, both in Australia and
South America, that in a quite recent period many of the largest and
most specialised forms have become extinct, while only the smaller types
have survived to our day; and a similar fact is to be observed in many
of the earlier geological epochs, a group progressing and reaching a
maximum of size or complexity and then dying out, or leaving at most but
few and pigmy representatives.
_Cause of Extinction of Large Animals._
Now there are several reasons for the repeated extinction of large
rather than of small animals. In the first place, animals of great bulk
require a proportionate supply of food, and any adverse change of
conditions would affect them more seriously than it would smaller
animals. In the next place, the extreme specialisation of many of these
large animals would render it less easy for them to be modified in any
new direction suited to changed conditions. Still more important,
perhaps, is the fact that very large animals always increase slowly as
compared with small ones--the elephant producing a single young one
every three years, while a rabbit may have a litter of seven or eight
young two or three times a year. Now the probability of favourable
variations will be in direct proportion to the population of the
species, and as the smaller animals are not only many hundred times more
numerous than the largest, but also increase perhaps a hundred times as
rapidly, they are able to become quickly modified by variation and
natural selection in harmony with changed conditions, while the large
and bulky species, being unable to vary quickly enough, are obliged to
succumb in the struggle for existence. As Professor Marsh well observes:
"In every vigorous primitive type which was destined to survive many
geological changes, there seems to have been a tendency to throw off
lateral branches, which became highly specialised and soon died out,
because they were unable to adapt themselves to new conditions." And he
goes on to show how the whole narrow path of the persistent Suilline
type, throughout the entire series of the American tertiaries, is
strewed with the remains of such ambitious offshoots, many of them
attaining the size of a rhinoceros; "while the typical pig, with an
obstinacy never lost, has held on in spite of catastrophes and
evolution, and still lives in America to-day."
_Indications of General Progression in Plants and Animals._
One of the most powerful arguments formerly adduced against evolution
was, that geology afforded no evidence of the gradual development of
organic forms, but that whole tribes and classes appeared suddenly at
definite epochs, and often in great variety and exhibiting a very
perfect organisation. The mammalia, for example, were long thought to
have first appeared in Tertiary times, where they are represented in
some of the earlier deposits by all the great divisions of the class
fully developed--carnivora, rodents, insectivora, marsupials, and even
the perissodactyle and artiodactyle divisions of the ungulata--as
clearly defined as at the present day. The discovery in 1818 of a single
lower jaw in the Stonesfield Slate of Oxfordshire hardly threw doubt on
the generalisation, since either its mammalian character was denied, or
the geological position of the strata, in which it was found, was held
to have been erroneously determined. But since then, at intervals of
many years, other remains of mammalia have been discovered in the
Secondary strata, ranging from the Upper Oolite to the Upper Trias both
in Europe and the United States, and one even (Tritylodon) in the Trias
of South Africa. All these are either marsupials, or of some still lower
type of mammalia; but they consist of many distinct forms classed in
about twenty genera. Nevertheless, a great gap still exists between
these mammals and those of the Tertiary strata, since no mammal of any
kind has been found in any part of the Cretaceous formation, although in
several of its subdivisions abundance of land plants, freshwater shells,
and air-breathing reptiles have been discovered. So with fishes. In the
last century none had been obtained lower than the Carboniferous
formation; thirty years later they were found to be very abundant in the
Devonian rocks, and later still they were discovered in the Upper Ludlow
and Lower Ludlow beds of the Silurian formation.
We thus see that such sudden appearances are deceptive, and are, in
fact, only what we ought to expect from the known imperfection of the
geological record. The conditions favourable to the fossilisation of any
group of animals occur comparatively rarely, and only in very limited
areas; while the conditions essential for their permanent preservation
in the rocks, amid all the destruction caused by denudation or
metamorphism, are still more exceptional. And when they are thus
preserved to our day, the particular part of the rocks in which they lie
hidden may not be on the surface but buried down deep under other
strata, and may thus, except in the case of mineral-bearing deposits, be
altogether out of our reach. Then, again, how large a proportion of the
earth consists of wild and uncivilised regions in which no exploration
of the rocks has been yet made, so that whether we shall find the
fossilised remains of any particular group of animals which lived during
a limited period of the earth's history, and in a limited area, depends
upon at least a fivefold combination of chances. Now, if we take each of
these chances separately as only ten to one against us (and some are
certainly more than this), then the actual chance against our finding
the fossil remains, say of any one order of mammalia, or of land plants,
at any particular geological horizon, will be about a hundred thousand
to one.
It may be said, if the chances are so great, how is it that we find such
immense numbers of fossil species exceeding in number, in some groups,
all those that are now living? But this is exactly what we should
expect, because the number of species of organisms that have ever lived
upon the earth, since the earliest geological times, will probably be
many hundred times greater than those now existing of which we have any
knowledge; and hence the enormous gaps and chasms in the geological
record of extinct forms is not to be wondered at. Yet, notwithstanding
these chasms in our knowledge, if evolution is true, there ought to have
been, on the whole, progression in all the chief types of life. The
higher and more specialised forms should have come into existence later
than the lower and more generalised forms; and however fragmentary the
portions we possess of the whole tree of life upon the earth, they ought
to show us broadly that such a progressive evolution has taken place. We
have seen that in some special groups, already referred to, such a
progression is clearly visible, and we will now cast a hasty glance over
the entire series of fossil forms, in order to see if a similar
progression is manifested by them as a whole.
_The Progressive Development of Plants._
Ever since fossil plants have been collected and studied, the broad fact
has been apparent that the early plants--those of the Coal
formation--were mainly cryptogamous, while in the Tertiary deposits the
higher flowering plants prevailed. In the intermediate secondary epoch
the gymnosperms--cycads and coniferae--formed a prominent part of the
vegetation, and as these have usually been held to be a kind of
transition form between the flowerless and flowering plants, the
geological succession has always, broadly speaking, been in accordance
with the theory of evolution. Beyond this, however, the facts were very
puzzling. The highest cryptogams--ferns, lycopods, and
equisetaceae--appeared suddenly, and in immense profusion in the Coal
formation, at which period they attained a development they have never
since surpassed or even equalled; while the highest plants--the
dicotyledonous and monocotyledonous angiosperms--which now form the bulk
of the vegetation of the world, and exhibit the most wonderful
modifications of form and structure, were almost unknown till the
Tertiary period, when they suddenly appeared in full development, and,
for the most part, under the same generic forms as now exist.
During the latter half of the present century, however, great additions
have been made to our knowledge of fossil plants; and although there
are still indications of vast gaps in our knowledge, due, no doubt, to
the very exceptional conditions required for the preservation of plant
remains, we now possess evidence of a more continuous development of the
various types of vegetation. According to Mr. Lester F. Ward, between
8000 and 9000 species of fossil plants have been described or indicated;
and, owing to the careful study of the nervation of leaves, a large
number of these are referable to their proper orders or genera, and
therefore give us some notion--which, though very imperfect, is probably
accurate in its main outlines--of the progressive development of
vegetation on the earth.[191] The following is a summary of the facts as
given by Mr. Ward:--
The lowest forms of vegetable life--the cellular plants--have been found
in Lower Silurian deposits in the form of three species of marine algae;
and in the whole Silurian formation fifty species have been recognised.
We cannot for a moment suppose, however, that this indicates the first
appearance of vegetable life upon the earth, for in these same Lower
Silurian beds the more highly organised vascular cryptogams appear in
the form of rhizocarps--plants allied to Marsilea and Azolla,--and a
very little higher, ferns, lycopods, and even conifers appear. We have
indications, however, of a still more ancient vegetation, in the
carbonaceous shales and thick beds of graphite far down in the Middle
Laurentian, since there is no other known agency than the vegetable cell
by means of which carbon can be extracted from the atmosphere and fixed
in the solid state. These great beds of graphite, therefore, imply the
existence of abundance of vegetable life at the very commencement of the
era of which we have any geological record.[192]
Ferns, as already stated, begin in the Middle Silurian formation with
the Eopteris Morrieri. In the Devonian, we have 79 species, in the
Carboniferous 627, and in the Permian 186 species; after which fossil
ferns diminish greatly, though they are found in every formation; and
the fact that fully 3000 living species are known, while the richest
portion of the Tertiary in fossil plants--the Miocene--- has only
produced 87 species, will serve to indicate the extreme imperfection of
the geological record.
The Equisetaceae (horsetails) which also first appear in the Silurian and
reach their maximum development in the Coal formation, are, in all
succeeding formations, far less numerous than ferns, and only thirty
living species are known. Lycopodiaceae, though still more abundant in
the Coal formation, are very rarely found in any succeeding deposit,
though the living species are tolerably numerous, about 500 having been
described. As we cannot suppose them to have really diminished and then
increased again in this extraordinary manner, we have another indication
of the exceptional nature of plant preservation and the extreme and
erratic character of the imperfection of the record.
Passing now to the next higher division of plants--the gymnosperms--we
find Coniferae appearing in the Upper Silurian, becoming tolerably
abundant in the Devonian, and reaching a maximum in the Carboniferous,
from which formation more than 300 species are known, equal to the
number recorded as now living. They occur in all succeeding formations,
being abundant in the Oolite, and excessively so in the Miocene, from
which 250 species have been described. The allied family of gymnosperms,
the Cycadaceae, first appear in the Carboniferous era, but very
scantily; are most abundant in the Oolite, from which formation 116
species are known, and then steadily diminish to the Tertiary, although
there are seventy-five living species.
We now come to the true flowering plants, and we first meet with
monocotyledons in the Carboniferous and Permian formations. The
character of these fossils was long disputed, but is now believed to be
well established; and the sub-class continues to be present in small
numbers in all succeeding deposits, becoming rather plentiful in the
Upper Cretaceous, and very abundant in the Eocene and Miocene. In the
latter formation 272 species have been discovered; but the 116 species
in the Eocene form a larger proportion of the total vegetation of the
period.
True dicotyledons appear very much later, in the Cretaceous period, and
only in its upper division, if we except a single species from the
Urgonian beds of Greenland. The remarkable thing is that we here find
the sub-class fully developed and in great luxuriance of types, all the
three divisions--Apetalae, Polypetalae, and Gamopetalae--being
represented, with a total of no less than 770 species. Among them are
such familiar forms as the poplar, the birch, the beech, the sycamore,
and the oak; as well as the fig, the true laurel, the sassafras, the
persimmon, the maple, the walnut, the magnolia, and even the apple and
the plum tribes. Passing on to the Tertiary period the numbers increase,
till they reach their maximum in the Miocene, where more than 2000
species of dicotyledons have been discovered. Among these the
proportionate number of the higher gamopetalae has slightly increased,
but is considerably less than at the present day.
_Possible Cause of sudden late Appearance of Exogens._
The sudden appearance of fully developed exogenous flowering plants in
the Cretaceous period is very analogous to the equally sudden appearance
of all the chief types of placental mammalia in the Eocene; and in both
cases we must feel sure that this suddenness is only apparent, due to
unknown conditions which have prevented their preservation (or their
discovery) in earlier formations. The case of the dicotyledonous plants
is in some respects the most extraordinary, because in the earlier
Mesozoic formations we appear to have a fair representation of the flora
of the period, including such varied forms as ferns, equisetums, cycads,
conifers, and monocotyledons. The only hint at an explanation of this
anomaly has been given by Mr. Ball, who supposes that all these groups
inhabited the lowlands, where there was not only excessive heat and
moisture, but also a superabundance of carbonic acid in the
atmosphere--conditions under which these groups had been developed, but
which were prejudicial to the dicotyledons. These latter are supposed to
have originated on the high table-lands and mountain ranges, in a rarer
and drier atmosphere in which the quantity of carbonic acid gas was much
less; and any deposits formed in lake beds at high altitudes and at such
a remote epoch have been destroyed by denudation, and hence we have no
record of their existence.[193]
During a few weeks spent recently in the Rocky Mountains, I was struck
by the great scarcity of monocotyledons and ferns in comparison with
dicotyledons--a scarcity due apparently to the dryness and rarity of the
atmosphere favouring the higher groups. If we compare Coulter's _Rocky
Mountain Botany_ with Gray's _Botany of the Northern (East) United
States_, we have two areas which differ chiefly in the points of
altitude and atmospheric moisture. Unfortunately, in neither of these
works are the species consecutively numbered; but by taking the pages
occupied by the two divisions of dicotyledons on the one hand,
monocotyledons and ferns on the other, we can obtain a good
approximation. In this way we find that in the flora of the
North-Eastern States the monocotyledons and ferns are to the
dicotyledons in the proportion of 45 to 100; in the Rocky Mountains they
are in the proportion of only 34 to 100; while if we take an exclusively
Alpine flora, as given by Mr. Ball, there are not one-fifth as many
monocotyledons as dicotyledons. These facts show that even at the
present day elevated plateaux and mountains are more favourable to
dicotyledons than to monocotyledons, and we may, therefore, well suppose
that the former originated within such elevated areas, and were for long
ages confined to them. It is interesting to note that their richest
early remains have been found in the central regions of the North
American continent, where they now, proportionally, most abound, and
where the conditions of altitude and a dry atmosphere were probably
present at a very early period.
[Illustration: FIG. 34.--Diagram illustrating the Geological
Distribution of Plants.]
The diagram (Fig. 34), slightly modified from one given by Mr. Ward,
will illustrate our present knowledge of the development of the
vegetable kingdom in geological time. The shaded vertical bands exhibit
the proportions of the fossil forms actually discovered, while the
outline extensions are intended to show what we may fairly presume to
have been the approximate periods of origin, and progressive increase of
the number of species, of the chief divisions of the vegetable kingdom.
These seem to accord fairly well with their respective grades of
development, and thus offer no obstacle to the acceptance of the belief
in their progressive evolution.
_Geological Distribution of Insects._
The marvellous development of insects into such an endless variety of
forms, their extreme specialisation, and their adaptation to almost
every possible condition of life, would almost necessarily imply an
extreme antiquity. Owing, however, to their small size, their lightness,
and their usually aerial habits, no class of animals has been so
scantily preserved in the rocks; and it is only recently that the whole
of the scattered material relating to fossil insects and their allies
have been brought together by Mr. Samuel H. Scudder of Boston, and we
have thus learned their bearing on the theory of evolution.[194]
The most striking fact which presents itself on a glance at the
distribution of fossil insects, is the completeness of the
representation of all the chief types far back in the Secondary period,
at which time many of the existing families appear to have been
perfectly differentiated. Thus in the Lias we find dragonflies
"apparently as highly specialised as to-day, no less than four tribes
being present." Of beetles we have undoubted Curculionidae from the Lias
and Trias; Chrysomelidae in the same deposits; Cerambycidae in the
Oolites; Scarabaeidae in the Lias; Buprestidae in the Trias; Elateridae,
Trogositidae, and Nitidulidae in the Lias; Staphylinidae in the English
Purbecks; while Hydrophilidae, Gyrinidae, and Carabidae occur in the
Lias. All these forms are well represented, but there are many other
families doubtfully identified in equally ancient rocks. Diptera of the
families Empidae, Asilidae, and Tipulidae have been found as far back as
the Lias. Of Lepidoptera, Sphingidae and Tineidae have been found in
the Oolite; while ants, representing the highly specialised Hymenoptera,
have occurred in the Purbeck and Lias.
This remarkable identity of the families of very ancient with those of
existing insects is quite comparable with the apparently sudden
appearance of existing genera of trees in the Cretaceous epoch. In both
cases we feel certain that we must go very much farther back in order to
find the ancestral forms from which they were developed, and that at any
moment some fresh discovery may revolutionise our ideas as to the
antiquity of certain groups. Such a discovery was made while Mr.
Scudder's work was passing through the press. Up to that date all the
existing orders of true insects appeared to have originated in the
Trias, the alleged moth and beetle of the Coal formation having been
incorrectly determined. But now, undoubted remains of beetles have been
found in the Coal measures of Silesia, thus supporting the
interpretation of the borings in carboniferous trees as having been made
by insects of this order, and carrying back this highly specialised form
of insect life well into Palaeozoic times. Such a discovery renders all
speculation as to the origin of true insects premature, because we may
feel sure that all the other orders of insects, except perhaps
hymenoptera and lepidoptera, were contemporaneous with the highly
specialised beetles.
The less highly organised terrestrial arthropoda--the Arachnida and
Myriapoda--are, as might be expected, much more ancient. A fossil spider
has been found in the Carboniferous, and scorpions in the Upper Silurian
rocks of Scotland, Sweden, and the United States. Myriapoda have been
found abundantly in the Carboniferous and Devonian formations; but all
are of extinct orders, exhibiting a more generalised structure than
living forms.
Much more extraordinary, however, is the presence in the Palaeozoic
formations of ancestral forms of true insects, termed by Mr. Scudder
Palaeodictyoptera. They consist of generalised cockroaches and
walking-stick insects (Orthopteroidea); ancient mayflies and allied
forms, of which there are six families and more than thirty genera
(Neuropteroidea); three genera of Hemipteroidea resembling various
Homoptera and Hemiptera, mostly from the Carboniferous formation, a few
from the Devonian, and one ancestral cockroach (Palaeoblattina) from
the Middle Silurian sandstone of France. If this occurrence of a true
hexapod insect from the Middle Silurian be really established, taken in
connection with the well-defined Coleoptera from the Carboniferous, the
origin of the entire group of terrestrial arthropoda is necessarily
thrown back into the Cambrian epoch, if not earlier. And this cannot be
considered improbable in view of the highly differentiated land
plants--ferns, equisetums, and lycopods--in the Middle or Lower
Silurian, and even a conifer (Cordaites Robbii) in the Upper Silurian;
while the beds of graphite in the Laurentian were probably formed from
terrestrial vegetation.
On the whole, then, we may affirm that, although the geological record
of the insect life of the earth is exceptionally imperfect, it yet
decidedly supports the evolution hypothesis. The most specialised order,
Lepidoptera, is the most recent, only dating back to the Oolite; the
Hymenoptera, Diptera, and Homoptera go as far as the Lias; while the
Orthoptera and Neuroptera extend to the Trias. The recent discovery of
Coleoptera in the Carboniferous shows, however, that the preceding
limits are not absolute, and will probably soon be overpassed. Only the
more generalised ancestral forms of winged insects have been traced back
to Silurian time, and along with them the less highly organised
scorpions; facts which serve to show us the extreme imperfection of our
knowledge, and indicate possibilities of a world of terrestrial life in
the remotest Palaeozoic times.
_Geological Succession of Vertebrata._
The lowest forms of vertebrates are the fishes, and these appear first
in the geological record in the Upper Silurian formation. The most
ancient known fish is a Pteraspis, one of the bucklered ganoids or
plated fishes--by no means a very low type--allied to the sturgeon
(Accipenser) and alligator-gar (Lepidosteus), but, as a group, now
nearly extinct. Almost equally ancient are the sharks, which under
various forms still abound in our seas. We cannot suppose these to be
nearly the earliest fishes, especially as the two lowest orders, now
represented by the Amphioxus or lancelet and the lampreys, have not yet
been found fossil. The ganoids were greatly developed in the Devonian
era, and continued till the Cretaceous, when they gave way to the true
osseous fishes, which had first appeared in the Jurassic period, and
have continued to increase till the present day. This much later
appearance of the higher osseous fishes is quite in accordance with
evolution, although some of the very lowest forms, the lancelet and the
lampreys, together with the archaic ceratodus, have survived to our
time.
The Amphibia, represented by the extinct labyrinthodons, appear first in
the Carboniferous rocks, and these peculiar forms became extinct early
in the Secondary period. The labyrinthodons were, however, highly
specialised, and do not at all indicate the origin of the class, which
may be as ancient as the lower forms of fishes. Hardly any recognisable
remains of our existing groups--the frogs, toads, and salamanders--are
found before the Tertiary period, a fact which indicates the extreme
imperfection of the record as regards this class of animals.
True reptiles have not been found till we reach the Permian where
Prohatteria and Proterosaurus occur, the former closely allied to the
lizard-like Sphenodon of New Zealand, the latter having its nearest
allies in the same group of reptiles--Rhyncocephala, other forms of
which occur in the Trias. In this last-named formation the earliest
crocodiles--Phytosaurus (Belodon) and Stagonolepis occur, as well as the
earliest tortoises--Chelytherium, Proganochelys, and Psephoderma.[195]
Fossil serpents have been first found in the Cretaceous formation, but
the conditions for the preservation of these forms have evidently been
unfavourable, and the record is correspondingly incomplete. The marine
Plesiosauri and Ichthyosauri, the flying Pterodactyles, the terrestrial
Iguanodon of Europe, and the huge Atlantosaurus of Colorado--the largest
land animal that has ever lived upon the earth[196]--all belong to
special developments of the reptilian type which flourished during the
Secondary epoch, and then became extinct.
Birds are among the rarest of fossils, due, no doubt, to their aerial
habits removing them from the ordinary dangers of flood, bog, or ice
which overwhelm mammals and reptiles, and also to their small specific
gravity which keeps them floating on the surface of water till devoured.
Their remains were long confined to Tertiary deposits, where many living
genera and a few extinct forms have been found. The only birds yet known
from the older rocks are the toothed birds (Odontornithes) of the
Cretaceous beds of the United States, belonging to two distinct families
and many genera; a penguin-like form (Enaliornis) from the Upper
Greensand of Cambridge; and the well-known long-tailed Archaeopteryx
from the Upper Oolite of Bavaria. The record is thus imperfect and
fragmentary in the extreme; but it yet shows us, in the few birds
discovered in the older rocks, more primitive and generalised types,
while the Tertiary birds had already become specialised like those
living, and had lost both the teeth and the long vertebral tail, which
indicate reptilian affinities in the earlier Mammalia have been found,
as already stated, as far back as the Trias formation, in Europe in the
United States and in South Africa, all being very small, and belonging
either to the Marsupial order, or to some still lower and more
generalised type, out of which both Marsupials and Insectivora were
developed. Other allied forms have been found in the Lower and Upper
Oolite both of Europe and the United States. But there is then a great
gap in the whole Cretaceous formation, from which no mammal has been
obtained, although both in the Wealden and the Upper Chalk in Europe,
and in the Upper Cretaceous deposits of the United States an abundant
and well-preserved terrestrial flora has been discovered. Why no mammals
have left their remains here it is impossible to say. We can only
suppose that the limited areas in which land plants have been so
abundantly preserved, did not present the conditions which are needed
for the fossilisation and preservation of mammalian remains.
When we come to the Tertiary formation, we find mammals in abundance;
but a wonderful change has taken place. The obscure early types have
disappeared, and we discover in their place a whole series of forms
belonging to existing orders, and even sometimes to existing families.
Thus, in the Eocene we have remains of the opossum family; bats
apparently belonging to living genera; rodents allied to the South
American cavies and to dormice and squirrels; hoofed animals belonging
to the odd-toed and even-toed groups; and ancestral forms of cats,
civets, dogs, with a number of more generalised forms of carnivora.
Besides these there are whales, lemurs, and many strange ancestral forms
of proboscidea.[197]
The great diversity of forms and structures at so remote an epoch would
require for their development an amount of time, which, judging by the
changes that have occurred in other groups, would carry us back far into
the Mesozoic period. In order to understand why we have no record of
these changes in any part of the world, we must fall back upon some such
supposition as we made in the case of the dicotyledonous plants.
Perhaps, indeed, the two cases are really connected, and the upland
regions of the primeval world, which saw the development of our higher
vegetation, may have also afforded the theatre for the gradual
development of the varied mammalian types which surprise us by their
sudden appearance in Tertiary times.
[Illustration: GEOLOGICAL DISTRIBUTION OF MAMMALIA.]
Notwithstanding these irregularities and gaps in the record, the
accompanying table, summarising our actual knowledge of the geological
distribution of the five classes of vertebrata, exhibits a steady
progression from lower to higher types, excepting only the deficiency in
the bird record which is easily explained. The comparative perfection of
type in which each of these classes first appears, renders it certain
that the origin of each and all of them must be sought much farther back
than any records which have yet been discovered. The researches of
palaeontologists and embryologists indicate a reptilian origin for birds
and mammals, while reptiles and amphibia arose, perhaps independently,
from fishes.
_Concluding Remarks._
The brief review we have now taken of the more suggestive facts
presented by the geological succession of organic forms, is sufficient
to show that most, if not all, of the supposed difficulties which it
presents in the way of evolution, are due either to imperfections in the
geological record itself, or to our still very incomplete knowledge of
what is really recorded in the earth's crust. We learn, however, that
just as discovery progresses, gaps are filled up and difficulties
disappear; while, in the case of many individual groups, we have already
obtained all the evidence of progressive development that can reasonably
be expected. We conclude, therefore, that the geological difficulty has
now disappeared; and that this noble science, when properly understood,
affords clear and weighty evidence of evolution.
FOOTNOTES:
[Footnote 183: The reader who desires to understand this subject more
fully, should study chap. x. of the _Origin of Species_, and chap. xiv.
of Sir Charles Lyell's _Principles of Geology_.]
[Footnote 184: On "Stagonolepis Robertsoni and on the Evolution of the
Crocodilia," in _Q.J. of Geological Society_, 1875; and abstract in
_Nature_, vol. xii. p. 38.]
[Footnote 185: From a paper by Messrs. Scott and Osborne, "On the Origin
and Development of the Rhinoceros Group," read before the British
Association in 1883.]
[Footnote 186: American Addresses, pp. 73-76.]
[Footnote 187: Lecture on the Introduction and Succession of Vertebrate
Life in America, _Nature_, vol. xvi. p. 471.]
[Footnote 188: _Nature_, vol. xxv. p. 84.]
[Footnote 189: See _The Mammalia in their Relation to Primeval Times_,
p. 102.]
[Footnote 190: For a brief enumeration and description of these fossils,
see the author's _Geographical Distribution of Animals_, vol. i. p.
146.]
[Footnote 191: Sketch of Palaeobotany in Fifth Annual Report of U.S.
Geological Survey, 1883-84, pp. 363-452, with diagrams. Sir J. William
Dawson, speaking of the value of leaves for the determination of fossil
plants, says: "In my own experience I have often found determinations of
the leaves of trees confirmed by the discovery of their fruits or of the
structure of their stems. Thus, in the rich cretaceous plant-beds of the
Dunvegan series, we have beech-nuts associated in the same bed with
leaves referred to _Fagus_. In the Laramie beds I determined many years
ago nuts of the _Trapa_ or water-chestnut, and subsequently Lesquereux
found in beds in the United States leaves which he referred to the same
genus. Later, I found in collections made on the Red Deer River of
Canada my fruits and Lesquereux's leaves on the same slab. The presence
of trees of the genera _Carya_ and _Juglans_ in the same formation was
inferred from their leaves, and specimens have since been obtained of
silicified wood with the microscopic structure of the modern butternut.
Still we are willing to admit that determinations from leaves alone are
liable to doubt."--_The Geological History of Plants_, p. 196.]
[Footnote 192: Sir J. William Dawson's _Geological History of Plants_,
p. 18.]
[Footnote 193: "On the Origin of the Flora of the European Alps," _Proc.
of Roy. Geog. Society_, vol. i. (1879), pp. 564-588.]
[Footnote 194: Systematic Review of our Present Knowledge of Fossil
Insects, including Myriapods and Arachnids (_Bull. of U.S. Geol.
Survey_, No. 31, Washington, 1886).]
[Footnote 195: For the facts as to the early appearance of the above
named groups of reptiles I am indebted to Mr. E. Lydekker of the
Geological Department of the Natural History Museum.]
[Footnote 196: According to Professor Marsh this creature was 50 or 60
feet long, and when erect, at least 30 feet in height. It fed upon the
foliage of the mountain forests of the Cretaceous epoch, the remains of
which are preserved with it.]
[Footnote 197: For fuller details, see the author's _Geographical
Distribution of Animals_, and Heilprin's _Geographical and Geological
Distribution of Animals_.]