NATURAL SELECTION BY VARIATION AND SURVIVAL OF THE FITTEST
Effect of struggle for existence under unchanged conditions--The
effect under change of conditions--Divergence of character--In
insects--In birds--In mammalia--Divergence leads to a maximum of
life in each area--Closely allied species inhabit distinct
areas--Adaptation to conditions at various periods of life--The
continued existence of low forms of life--Extinction of low
types among the higher animals--Circumstances favourable to the
origin of new species--Probable origin of the dippers--The
importance of isolation--On the advance of organisation by
natural selection--Summary of the first five chapters.
In the preceding chapters we have accumulated a body of facts and
arguments which will enable us now to deal with the very core of our
subject--the formation of species by means of natural selection. We have
seen how tremendous is the struggle for existence always going on in
nature owing to the great powers of increase of all organisms; we have
ascertained the fact of variability extending to every part and organ,
each of which varies simultaneously and for the most part independently;
and we have seen that this variability is both large in its amount in
proportion to the size of each part, and usually affects a considerable
proportion of the individuals in the large and dominant species. And,
lastly, we have seen how similar variations, occurring in cultivated
plants and domestic animals, are capable of being perpetuated and
accumulated by artificial selection, till they have resulted in all the
wonderful varieties of our fruits, flowers, and vegetables, our domestic
animals and household pets, many of which differ from each other far
more in external characters, habits, and instincts than do species in a
state of nature. We have now to inquire whether there is any analogous
process in nature, by which wild animals and plants can be permanently
modified and new races or new species produced.
_Effect of Struggle for Existence under Unchanged Conditions._
Let us first consider what will be the effect of the struggle for
existence upon the animals and plants which we see around us, under
conditions which do not perceptibly vary from year to year or from
century to century. We have seen that every species is exposed to
numerous and varied dangers throughout its entire existence, and that it
is only by means of the exact adaptation of its organisation--including
its instincts and habits--to its surroundings that it is enabled to live
till it produces offspring which may take its place when it ceases to
exist. We have seen also that, of the whole annual increase only a very
small fraction survives; and though the survival in individual cases may
sometimes be due rather to accident than to any real superiority, yet we
cannot doubt that, in the long run, those survive which are best fitted
by their perfect organisation to escape the dangers that surround them.
This "survival of the fittest" is what Darwin termed "natural
selection," because it leads to the same results in nature as are
produced by man's selection among domestic animals and cultivated
plants. Its primary effect will, clearly, be to keep each species in the
most perfect health and vigour, with every part of its organisation in
full harmony with the conditions of its existence. It prevents any
possible deterioration in the organic world, and produces that
appearance of exuberant life and enjoyment, of health and beauty, that
affords us so much pleasure, and which might lead a superficial observer
to suppose that peace and quietude reigned throughout nature.
_The Effect under changed Conditions._
But the very same process which, so long as conditions remain
substantially the same, secures the continuance of each species of
animal or plant in its full perfection, will usually, under changed
conditions, bring about whatever change of structure or habits may be
necessitated by them. The changed conditions to which we refer are such
as we know have occurred throughout all geological time and in every
part of the world. Land and water have been continually shifting their
positions; some regions are undergoing subsidence with diminution of
area, others elevation with extension of area; dry land has been
converted into marshes, while marshes have been drained or have even
been elevated into plateaux. Climate too has changed again and again,
either through the elevation of mountains in high latitudes leading to
the accumulation of snow and ice, or by a change in the direction of
winds and ocean currents produced by the subsidence or elevation of
lands which connected continents and divided oceans. Again, along with
all these changes have come not less important changes in the
distribution of species. Vegetation has been greatly modified by changes
of climate and of altitude; while every union of lands before separated
has led to extensive migrations of animals into new countries,
disturbing the balance that before existed among its forms of life,
leading to the extermination of some species and the increase of others.
When such physical changes as these have taken place, it is evident that
many species must either become modified or cease to exist. When the
vegetation has changed in character the herbivorous animals must become
able to live on new and perhaps less nutritious food; while the change
from a damp to a dry climate may necessitate migration at certain
periods to escape destruction by drought. This will expose the species
to new dangers, and require special modifications of structure to meet
them. Greater swiftness, increased cunning, nocturnal habits, change of
colour, or the power of climbing trees and living for a time on their
foliage or fruit, may be the means adopted by different species to bring
themselves into harmony with the new conditions; and by the continued
survival of those individuals, only, which varied sufficiently in the
right direction, the necessary modifications of structure or of function
would be brought about, just as surely as man has been able to breed the
greyhound to hunt by sight and the foxhound by scent, or has produced
from the same wild plant such distinct forms as the cauliflower and the
brussels sprouts.
We will now consider the special characteristics of the changes in
species that are likely to be effected, and how far they agree with what
we observe in nature.
_Divergence of Character._
In species which have a wide range the struggle for existence will often
cause some individuals or groups of individuals to adopt new habits in
order to seize upon vacant places in nature where the struggle is less
severe. Some, living among extensive marshes, may adopt a more aquatic
mode of life; others, living where forests abound, may become more
arboreal. In either case we cannot doubt that the changes of structure
needed to adapt them to their new habits would soon be brought about,
because we know that variations in all the external organs and all their
separate parts are very abundant and are also considerable in amount.
That such divergence of character has actually occurred we have some
direct evidence. Mr. Darwin informs us that in the Catskill Mountains in
the United States there are two varieties of wolves, one with a light
greyhound-like form which pursues deer, the other more bulky with
shorter legs, which more frequently attacks sheep.[37] Another good
example is that of the insects in the island of Madeira, many of which
have either lost their wings or have had them so much reduced as to be
useless for flight, while the very same species on the continent of
Europe possess fully developed wings. In other cases the wingless
Madeira species are distinct from, but closely allied to, winged species
of Europe. The explanation of this change is, that Madeira, like many
oceanic islands in the temperate zone, is much exposed to sudden gales
of wind, and as most of the fertile land is on the coast, insects which
flew much would be very liable to be blown out to sea and lost. Year
after year, therefore, those individuals which had shorter wings, or
which used them least, were preserved; and thus, in time, terrestrial,
wingless, or imperfectly winged races or species have been produced.
That this is the true explanation of this singular fact is proved by
much corroborative evidence. There are some few flower-frequenting
insects in Madeira to whom wings are essential, and in these the wings
are somewhat larger than in the same species on the mainland. We thus
see that there is no general tendency to the abortion of wings in
Madeira, but that it is simply a case of adaptation to new conditions.
Those insects to whom wings were not absolutely essential escaped a
serious danger by not using them, and the wings therefore became reduced
or were completely lost. But when they were essential they were enlarged
and strengthened, so that the insect could battle against the winds and
save itself from destruction at sea. Many flying insects, not varying
fast enough, would be destroyed before they could establish themselves,
and thus we may explain the total absence from Madeira of several whole
families of winged insects which must have had many opportunities of
reaching the islands. Such are the large groups of the tiger-beetles
(Cicindelidae), the chafers (Melolonthidae), the click-beetles
(Elateridae), and many others.
But the most curious and striking confirmation of this portion of Mr.
Darwin's theory is afforded by the case of Kerguelen Island. This island
was visited by the _Transit of Venus_ expedition. It is one of the
stormiest places on the globe, being subject to almost perpetual gales,
while, there being no wood, it is almost entirely without shelter. The
Rev. A.E. Eaton, an experienced entomologist, was naturalist to the
expedition, and he assiduously collected the few insects that were to be
found. All were incapable of flight, and most of them entirely without
wings. They included a moth, several flies, and numerous beetles. As
these insects could hardly have reached the islands in a wingless state,
even if there were any other known land inhabited by them--which there
is not--we must assume that, like the Madeiran insects, they were
originally winged, and lost their power of flight because its possession
was injurious to them.
It is no doubt due to the same cause that some butterflies on small and
exposed islands have their wings reduced in size, as is strikingly the
case with the small tortoise-shell butterfly (Vanessa urticae)
inhabiting the Isle of Man, which is only about half the size of the
same species in England or Ireland; and Mr. Wollaston notes that Vanessa
callirhoe--a closely allied South European form of our red-admiral
butterfly--is permanently smaller in the small and bare island of Porto
Santo than in the larger and more wooded adjacent island of Madeira.
A very good example of comparatively recent divergence of character, in
accordance with new conditions of life, is afforded by our red grouse.
This bird, the Lagopus scoticus of naturalists, is entirely confined to
the British Isles. It is, however, very closely allied to the willow
grouse (Lagopus albus), a bird which ranges all over Europe, Northern
Asia, and North America, but which, unlike our species, changes to white
in winter. No difference in form or structure can be detected between
the two birds, but as they differ so decidedly in colour--our species
being usually rather darker in winter than in summer, while there are
also slight differences in the call-note and in habits,--the two species
are generally considered to be distinct. The differences, however, are
so clearly adaptations to changed conditions that we can hardly doubt
that, during the early part of the glacial period, when our islands were
united to the continent, our grouse was identical with that of the rest
of Europe. But when the cold passed away and our islands became
permanently separated from the mainland, with a mild and equable climate
and very little snow in winter, the change to white at that season
became hurtful, rendering the birds more conspicuous instead of serving
as a means of concealment. The colour was, therefore, gradually changed
by the process of variation and natural selection; and as the birds
obtained ample shelter among the heather which clothes so many of our
moorlands, it became useful for them to assimilate with its brown and
dusky stems and withered flowers rather than with the snow of the higher
mountains. An interesting confirmation of this change having really
occurred is afforded by the occasional occurrence in Scotland of birds
with a considerable amount of white in the winter plumage. This is
considered to be a case of reversion to the ancestral type, just as the
slaty colours and banded wings of the wild rock-pigeon sometimes
reappear in our fancy breeds of domestic pigeons.[38]
The principle of "divergence of character" pervades all nature from the
lowest groups to the highest, as may be well seen in the class of birds.
Among our native species we see it well marked in the different species
of titmice, pipits, and chats. The great titmouse (Parus major) by its
larger size and stronger bill is adapted to feed on larger insects, and
is even said sometimes to kill small and weak birds. The smaller and
weaker coal titmouse (Parus ater) has adopted a more vegetarian diet,
eating seeds as well as insects, and feeding on the ground as well as
among trees. The delicate little blue titmouse (Parus coeruleus), with
its very small bill, feeds on the minutest insects and grubs which it
extracts from crevices of bark and from the buds of fruit-trees. The
marsh titmouse, again (Parus palustris), has received its name from the
low and marshy localities it frequents; while the crested titmouse
(Parus cristatus) is a northern bird frequenting especially pine
forests, on the seeds of which trees it partially feeds. Then, again,
our three common pipits--the tree-pipit (Anthus arboreus), the
meadow-pipit (Anthus pratensis), and the rock-pipit or sea-lark (Anthus
obscurus) have each occupied a distinct place in nature to which they
have become specially adapted, as indicated by the different form and
size of the hind toe and claw in each species. So, the stone-chat
(Saxicola rubicola), the whin-chat (S. rubetra), and the wheat-ear (S.
oenanthe) are more or less divergent forms of one type, with
modifications in the shape of the wing, feet, and bill adapting them to
slightly different modes of life. The whin-chat is the smallest, and
frequents furzy commons, fields, and lowlands, feeding on worms,
insects, small molluscs, and berries; the stone-chat is next in size,
and is especially active and lively, frequenting heaths and uplands, and
is a permanent resident with us, the two other species being migrants;
while the larger and more conspicuous wheat-ear, besides feeding on
grubs, beetles, etc., is able to capture flying insects on the wing,
something after the manner of true flycatchers.
These examples sufficiently indicate how divergence of character has
acted, and has led to the adaptation of numerous allied species, each to
a more or less special mode of life, with the variety of food, of
habits, and of enemies which must necessarily accompany such diversity.
And when we extend our inquiries to higher groups we find the same
indications of divergence and special adaptation, often to a still more
marked extent. Thus we have the larger falcons, which prey upon birds,
while some of the smaller species, like the hobby (Falco subbuteo), live
largely on insects. The true falcons capture their prey in the air,
while the hawks usually seize it on or near the ground, feeding on
hares, rabbits, squirrels, grouse, pigeons, and poultry. Kites and
buzzards, on the other hand, seize their prey upon the ground, and the
former feed largely on reptiles and offal as well as on birds and
quadrupeds. Others have adopted fish as their chief food, and the osprey
snatches its prey from the water with as much facility as a gull or a
petrel; while the South American caracaras (Polyborus) have adopted the
habits of vultures and live altogether on carrion. In every great group
there is the same divergence of habits. There are ground-pigeons,
rock-pigeons, and wood-pigeons,--seed-eating pigeons and fruit-eating
pigeons; there are carrion-eating, insect-eating, and fruit-eating
crows. Even kingfishers are, some aquatic, some terrestrial in their
habits; some live on fish, some on insects, some on reptiles. Lastly,
among the primary divisions of birds we find a purely terrestrial
group--the Ratitae, including the ostriches, cassowaries, etc.; other
great groups, including the ducks, cormorants, gulls, penguins, etc.,
are aquatic; while the bulk of the Passerine birds are aerial and
arboreal. The same general facts can be detected in all other classes of
animals. In the mammalia, for example, we have in the common rat a
fish-eater and flesh-eater as well as a grain-eater, which has no doubt
helped to give it the power of spreading over the world and driving away
the native rats of other countries. Throughout the Rodent tribe we find
everywhere aquatic, terrestrial, and arboreal forms. In the weasel and
cat tribes some live more in trees, others on the ground; squirrels have
diverged into terrestrial, arboreal, and flying species; and finally, in
the bats we have a truly aerial, and in the whales a truly aquatic order
of mammals. We thus see that, beginning with different varieties of the
same species, we have allied species, genera, families, and orders, with
similarly divergent habits, and adaptations to different modes of life,
indicating some general principle in nature which has been operative in
the development of the organic world. But in order to be thus operative
it must be a generally useful principle, and Mr. Darwin has very clearly
shown us in what this utility consists.
_Divergence leads to a Maximum of Organic Forms in each Area._
Divergence of character has a double purpose and use. In the first place
it enables a species which is being overcome by rivals, or is in
process of extinction by enemies, to save itself by adopting new habits
or by occupying vacant places in nature. This is the immediate and
obvious effect of all the numerous examples of divergence of character
which we have pointed out. But there is another and less obvious result,
which is, that the greater the diversity in the organisms inhabiting a
country or district the greater will be the total amount of life that
can be supported there. Hence the continued action of the struggle for
existence will tend to bring about more and more diversity in each area,
which may be shown to be the case by several kinds of evidence. As an
example, a piece of turf, three feet by four in size, was found by Mr.
Darwin to contain twenty species of plants, and these twenty species
belonged to eighteen genera and to eight orders, showing how greatly
they differed from each other. Farmers find that a greater quantity of
hay is obtained from ground sown with a variety of genera of grasses,
clover, etc., than from similar land sown with one or two species only;
and the same principle applies to rotation of crops, plants differing
very widely from each other giving the best results. So, in small and
uniform islands, and in small ponds of fresh water, the plants and
insects, though few in number, are found to be wonderfully varied in
character.
The same principle is seen in the naturalisation of plants and animals
by man's agency in distant lands, for the species that thrive best and
establish themselves permanently are not only very varied among
themselves but differ greatly from the native inhabitants. Thus, in the
Northern United States there are, according to Dr. Asa Gray, 260
naturalised flowering plants which belong to no less than 162 genera;
and of these, 100 genera are not natives of the United States. So, in
Australia, the rabbit, though totally unlike any native animal, has
increased so much that it probably outnumbers in individuals all the
native mammals of the country; and in New Zealand the rabbit and the pig
have equally multiplied. Darwin remarks that this "advantage of
diversification of structure in the inhabitants of the same region is,
in fact, the same as that of the physiological division of labour in the
organs of the same body. No physiologist doubts that a stomach adapted
to digest vegetable matter alone, or flesh alone, draws more nutriment
from these substances. So, in the general economy of any land, the more
widely and perfectly the animals and plants are diversified for
different habits of life, so will a greater number of individuals be
capable of there supporting themselves."[39]
_The most closely allied Species inhabit distinct Areas._
One of the curious results of the general action of this principle in
nature is, that the most closely allied species--those whose differences
though often real and important are hardly perceptible to any one but a
naturalist--are usually not found in the same but in widely separated
countries. Thus, the nearest allies to our European golden plover are
found in North America and East Asia; the nearest ally of our European
jay is found in Japan, although there are several other species of jays
in Western Asia and North Africa; and though we have several species of
titmice in England they are not very closely allied to each other. The
form most akin to our blue tit is the azure tit of Central Asia (Parus
azureus); the Parus ledouci of Algeria is very near our coal tit, and
the Parus lugubris of South-Eastern Europe and Asia Minor is nearest to
our marsh tit. So, our four species of wild pigeons--the ring-dove,
stock-dove, rock-pigeon, and turtle-dove--are not closely allied to each
other, but each of them belongs, according to some ornithologists, to a
separate genus or subgenus, and has its nearest relatives in distant
parts of Asia and Africa. In mammalia the same thing occurs. Each
mountain region of Europe and Asia has usually its own species of wild
sheep and goat, and sometimes of antelope and deer; so that in each
region there is found the greatest diversity in this class of animals,
while the closest allies inhabit quite distinct and often distant areas.
In plants we find the same phenomenon prevalent. Distinct species of
columbine are found in Central Europe (Aguilegia vulgaris), in Eastern
Europe, and Siberia (A. glandulosa), in the Alps (A. Alpina), in the
Pyrenees (A. pyrenaiea), in the Greek mountains (A. ottonis), and in
Corsica (A. Bernardi), but rarely are two species found in the same
area. So, each part of the world has its own peculiar forms of pines,
firs, and cedars, but the closely allied species or varieties are in
almost every case inhabitants of distinct areas. Examples are the deodar
of the Himalayas, the cedar of Lebanon, and that of North Africa, all
very closely allied but confined to distinct areas; and the numerous
closely allied species of true pine (genus Pinus), which almost always
inhabit different countries or occupy different stations. We will now
consider some other modes in which natural selection will act, to adapt
organisms to changed conditions.
_Adaptation to Conditions at Various Periods of Life._
It is found, that, in domestic animals and cultivated plants, variations
occurring at any one period of life reappear in the offspring at the
same period, and can be perpetuated and increased by selection without
modifying other parts of the organisation. Thus, variations in the
caterpillar or the cocoon of the silkworm, in the eggs of poultry, and
in the seeds or young shoots of many culinary vegetables, have been
accumulated till those parts have become greatly modified and, for man's
purposes, improved. Owing to this fact it is easy for organisms to
become so modified as to avoid dangers that occur at any one period of
life. Thus it is that so many seeds have become adapted to various modes
of dissemination or protection. Some are winged, or have down or hairs
attached to them, so as to enable them to be carried long distances in
the air; others have curious hooks and prickles, which cause them to be
attached firmly to the fur of mammals or the feathers of birds; while
others are buried within sweet or juicy and brightly coloured fruits,
which are seen and devoured by birds, the hard smooth seeds passing
through their bodies in a fit state for germination. In the struggle for
existence it must benefit a plant to have increased means of dispersing
its seeds, and of thus having young plants produced in a greater variety
of soils, aspects, and surroundings, with a greater chance of some of
them escaping their numerous enemies and arriving at maturity. The
various differences referred to would, therefore, be brought about by
variation and survival of the fittest, just as surely as the length and
quality of cotton on the seed of the cotton-plant have been increased
by man's selection.
The larvae of insects have thus been wonderfully modified in order to
escape the numerous enemies to whose attacks they are exposed at this
period of their existence. Their colours and markings have become
marvellously adapted to conceal them among the foliage of the plant they
live upon, and this colour often changes completely after the last
moult, when the creature has to descend to the ground for its change to
the pupa state, during which period a brown instead of a green colour is
protective. Others have acquired curious attitudes and large ocelli,
which cause them to resemble the head of some reptile, or they have
curious horns or coloured ejectile processes which frighten away
enemies; while a great number have acquired secretions which render them
offensive to the taste of their enemies, and these are always adorned
with very conspicuous markings or brilliant colours, which serve as a
sign of inedibility and prevent their being needlessly attacked. This,
however, is a portion of the very large subject of organic colour and
marking, which will be fully discussed and illustrated in a separate
chapter.
In this way every possible modification of an animal or plant, whether
in colour, form, structure, or habits, which would be serviceable to it
or to its progeny at any period of its existence, may be readily brought
about. There are some curious organs which are used only once in a
creature's life, but which are yet essential to its existence, and thus
have very much the appearance of design by an intelligent designer. Such
are, the great jaws possessed by some insects, used exclusively for
opening the cocoon, and the hard tip to the beak of unhatched birds used
for breaking the eggshell. The increase in thickness or hardness of the
cocoons or the eggs being useful for protection against enemies or to
avoid accidents, it is probable that the change has been very gradual,
because it would be constantly checked by the necessity for a
corresponding change in the young insects or birds enabling them to
overcome the additional obstacle of a tougher cocoon or a harder
eggshell. As we have seen, however, that every part of the organism
appears to be varying independently, at the same time, though to
different amounts, there seems no reason to believe that the necessity
for two or more coincident variations would prevent the required change
from taking place.
_The Continued Existence of Low Forms of Life._
Since species are continually undergoing modifications giving them some
superiority over other species or enabling them to occupy fresh places
in nature, it may be asked--Why do any low forms continue to exist? Why
have they not long since been improved and developed into higher forms?
The answer, probably, is, that these low forms occupy places in nature
which cannot be filled by higher forms, and that they have few or no
competitors; they therefore continue to exist. Thus, earthworms are
adapted to their mode of life better than they would be if more highly
organised. So, in the ocean, the minute foraminifera and infusoria, and
the larger sponges and corals, occupy places which more highly developed
creatures could not fill. They form, as it were, the base of the great
structure of animal life, on which the next higher forms rest; and
though in the course of ages they may undergo some changes, and
diversification of form and structure, in accordance with changed
conditions, their essential nature has probably remained the same from
the very dawn of life on the earth. The low aquatic diatomaceae and
confervae, together with the lowest fungi and lichens, occupy a similar
position in the vegetable kingdom, filling places in nature which would
be left vacant if only highly organised plants existed. There is,
therefore, no motive power to destroy or seriously to modify them; and
they have thus probably persisted, under slightly varying forms, through
all geological time.
_Extinction of Lower Types among the Higher Animals._
So soon; however, as we approach the higher and more fully developed
groups, we see indications of the often repeated extinction of lower by
higher forms. This is shown by the great gaps that separate the
mammalia, birds, reptiles, and fishes from each other; while the lowest
forms of each are always few in number and confined to limited areas.
Such are the lowest mammals--the echidna and ornithorhynchus of
Australia; the lowest birds--the apteryx of New Zealand and the
cassowaries of the New Guinea region; while the lowest fish--the
amphioxus or lancelet, is completely isolated, and has apparently
survived only by its habit of burrowing in the sand. The great
distinctness of the carnivora, ruminants, rodents, whales, bats, and
other orders of mammalia; of the accipitres, pigeons, and parrots, among
birds; and of the beetles, bees, flies, and moths, among insects, all
indicate an enormous amount of extinction among the comparatively low
forms by which, on any theory of evolution, these higher and more
specialised groups must have been preceded.
_Circumstances favourable to the Origin of New Species by Natural
Selection._
We have already seen that, when there is no change in the physical or
organic conditions of a country, the effect of natural selection is to
keep all the species inhabiting it in a state of perfect health and full
development, and to preserve the balance that already exists between the
different groups of organisms. But, whenever the physical or organic
conditions change, to however small an extent, some corresponding change
will be produced in the flora and fauna, since, considering the severe
struggle for existence and the complex relations of the various
organisms, it is hardly possible that the change should not be
beneficial to some species and hurtful to others. The most common
effect, therefore, will be that some species will increase and others
will diminish; and in cases where a species was already small in numbers
a further diminution might lead to extinction. This would afford room
for the increase of other species, and thus a considerable readjustment
of the proportions of the several species might take place. When,
however, the change was of a more important character, directly
affecting the existence of many species so as to render it difficult for
them to maintain themselves without some considerable change in
structure or habits, that change would, in some cases, be brought about
by variation and natural selection, and thus new varieties or new
species might be formed. We have to consider, then, which are the
species that would be most likely to be so modified, while others, not
becoming modified, would succumb to the changed conditions and become
extinct.
The most important condition of all is, undoubtedly, that variations
should occur of sufficient amount, of a sufficiently diverse character,
and in a large number of individuals, so as to afford ample materials
for natural selection to act upon; and this, we have seen, does occur in
most, if not in all, large, wide-ranging, and dominant species. From
some of these, therefore, the new species adapted to the changed
conditions would usually be derived; and this would especially be the
case when the change of conditions was rather rapid, and when a
correspondingly rapid modification could alone save some species from
extinction. But when the change was very gradual, then even less
abundant and less widely distributed species might become modified into
new forms, more especially if the extinction of many of the rarer
species left vacant places in the economy of nature.
_Probable Origin of the Dippers._
An excellent example of how a limited group of species has been able to
maintain itself by adaptation to one of these "vacant places" in nature,
is afforded by the curious little birds called dippers or water-ouzels,
forming the genus Cinclus and the family Cinclidae of naturalists. These
birds are something like small thrushes, with very short wings and tail,
and very dense plumage. They frequent, exclusively, mountain torrents in
the northern hemisphere, and obtain their food entirely in the water,
consisting, as it does, of water-beetles, caddis-worms and other
insect-larvae, as well as numerous small freshwater shells. These birds,
although not far removed in structure from thrushes and wrens, have the
extraordinary power of flying under water; for such, according to the
best observers, is their process of diving in search of their prey,
their dense and somewhat fibrous plumage retaining so much air that the
water is prevented from touching their bodies or even from wetting their
feathers to any great extent. Their powerful feet and long curved claws
enable them to hold on to stones at the bottom, and thus to retain their
position while picking up insects, shells, etc. As they frequent
chiefly the most rapid and boisterous torrents, among rocks, waterfalls,
and huge boulders, the water is never frozen over, and they are thus
able to live during the severest winters. Only a very few species of
dipper are known, all those of the old world being so closely allied to
our British bird that some ornithologists consider them to be merely
local races of one species; while in North America and the northern
Andes there are two other species.
Here then we have a bird, which, in its whole structure, shows a close
affinity to the smaller typical perching birds, but which has departed
from all its allies in its habits and mode of life, and has secured for
itself a place in nature where it has few competitors and few enemies.
We may well suppose, that, at some remote period, a bird which was
perhaps the common and more generalised ancestor of most of our
thrushes, warblers, wrens, etc., had spread widely over the great
northern continent, and had given rise to numerous varieties adapted to
special conditions of life. Among these some took to feeding on the
borders of clear streams, picking out such larvae and molluscs as they
could reach in shallow water. When food became scarce they would attempt
to pick them out of deeper and deeper water, and while doing this in
cold weather many would become frozen and starved. But any which
possessed denser and more hairy plumage than usual, which was able to
keep out the water, would survive; and thus a race would be formed which
would depend more and more on this kind of food. Then, following up the
frozen streams into the mountains, they would be able to live there
during the winter; and as such places afforded them much protection from
enemies and ample shelter for their nests and young, further adaptations
would occur, till the wonderful power of diving and flying under water
was acquired by a true land-bird.
That such habits might be acquired under stress of need is rendered
highly probable by the facts stated by the well-known American
naturalist, Dr. Abbott. He says that "the water-thrushes (Seiurus sp.)
all wade in water, and often, seeing minute mollusca on the bottom of
the stream, plunge both head and neck beneath the surface, so that
often, for several seconds, a large part of the body is submerged. Now
these birds still have the plumage pervious to water, and so are liable
to be drenched and sodden; but they have also the faculty of giving
these drenched feathers such a good shaking that flight is practicable a
moment after leaving the water. Certainly the water-thrushes (Seiurus
ludovicianus, S. auricapillus, and S. noveboracensis) have taken many
preliminary steps to becoming as aquatic as the dipper; and the
winter-wren, and even the Maryland yellow-throat are not far
behind."[40]
Another curious example of the way in which species have been modified
to occupy new places in nature, is afforded by the various animals which
inhabit the water-vessels formed by the leaves of many epiphytal species
of Bromelia. Fritz Müller has described a caddis-fly larva which lives
among these leaves, and which has been modified in the pupa state in
accordance with its surroundings. The pupae of caddis-flies inhabiting
streams have fringes of hair on the tarsi to enable them to reach the
surface on leaving their cases. But in the species inhabiting bromelia
leaves there is no need for swimming, and accordingly we find the tarsi
entirely bare. In the same plants are found curious little Entomostraca,
very abundant there but found nowhere else. These form a new genus, but
are most nearly allied to Cythere, a marine type. It is believed that
the transmission of this species from one tree to another must be
effected by the young crustacea, which are very minute, clinging to
beetles, many of which, both terrestrial and aquatic, also inhabit the
bromelia leaves; and as some water-beetles are known to frequent the
sea, it is perhaps by these means that the first emigrants established
themselves in this strange new abode. Bromeliae are often very abundant
on trees growing on the water's edge, and this would facilitate the
transition from a marine to an arboreal habitat. Fritz Müller has also
found, among the bromelia leaves, a small frog bearing its eggs on its
back, and having some other peculiarities of structure. Several
beautiful little aquatic plants of the genus Utricularia or bladder-wort
also inhabit bromelia leaves; and these send runners out to neighbouring
plants and thus spread themselves with great rapidity.
_The Importance of Isolation._
Isolation is no doubt an important aid to natural selection, as shown by
the fact that islands so often present a number of peculiar species; and
the same thing is seen on the two sides of a great mountain range or on
opposite coasts of a continent. The importance of isolation is twofold.
In the first place, it leads to a body of individuals of each species
being limited in their range and thus subjected to uniform conditions
for long spaces of time. Both the direct action of the environment and
the natural selection of such varieties only as are suited to the
conditions, will, therefore, be able to produce their full effect. In
the second place, the process of change will not be interfered with by
intercrossing with other individuals which are becoming adapted to
somewhat different conditions in an adjacent area. But this question of
the swamping effects of intercrossing will be considered in another
chapter.
Mr. Darwin was of opinion that, on the whole, the largeness of the area
occupied by a species was of more importance than isolation, as a factor
in the production of new species, and in this I quite agree with him. It
must, too, be remembered, that isolation will often be produced in a
continuous area whenever a species becomes modified in accordance with
varied conditions or diverging habits. For example, a wide-ranging
species may in the northern and colder part of its area become modified
in one direction, and in the southern part in another direction; and
though for a long time an intermediate form may continue to exist in the
intervening area, this will be likely soon to die out, both because its
numbers will be small, and it will be more or less pressed upon in
varying seasons by the modified varieties, each better able to endure
extremes of climate. So, when one portion of a terrestrial species takes
to a more arboreal or to a more aquatic mode of life, the change of
habit itself leads to the isolation of each portion. Again, as will be
more fully explained in a future chapter, any difference of habits or of
haunts usually leads to some modification of colour or marking, as a
means of concealment from enemies; and there is reason to believe that
this difference will be intensified by natural selection as a means of
identification and recognition by members of the same variety or
incipient species. It has also been observed that each differently
coloured variety of wild animals, or of domesticated animals which have
run wild, keep together, and refuse to pair with individuals of the
other colours; and this must of itself act to keep the races separate as
completely as physical isolation.
_On the Advance of Organisation by Natural Selection._
As natural selection acts solely by the preservation of useful
variations, or those which are beneficial to the organism under the
conditions to which it is exposed, the result must necessarily be that
each species or group tends to become more and more improved in relation
to its conditions. Hence we should expect that the larger groups in each
class of animals and plants--those which have persisted and have been
abundant throughout geological ages--would, almost necessarily, have
arrived at a high degree of organisation, both physical and mental.
Illustrations of this are to be seen everywhere. Among mammalia we have
the carnivora, which from Eocene times have been becoming more and more
specialised, till they have culminated in the cat and dog tribes, which
have reached a degree of perfection both in structure and intelligence
fully equal to that of any other animals. In another line of
development, the herbivora have been specialised for living solely on
vegetable food till they have culminated in the sheep, the cattle, the
deer, and the antelopes. The horse tribe, commencing with an early
four-toed ancestor in the Eocene age, has increased in size and in
perfect adaptation of feet and teeth to a life on open plains, and has
reached its highest perfection in the horse, the ass, and the zebra. In
birds, also, we see an advance from the imperfect tooth-billed and
reptile-tailed birds of the secondary epoch, to the wonderfully
developed falcons, crows, and swallows of our time. So, the ferns,
lycopods, conifers, and monocotyledons of the palaeozoic and mesozoic
rocks, have developed into the marvellous wealth of forms of the higher
dicotyledons that now adorn the earth.
But this remarkable advance in the higher and larger groups does not
imply any universal law of progress in organisation, because we have at
the same time numerous examples (as has been already pointed out) of the
persistence of lowly organised forms, and also of absolute degradation
or degeneration. Serpents, for example, have been developed from some
lizard-like type which has lost its limbs; and though this loss has
enabled them to occupy fresh places in nature and to increase and
flourish to a marvellous extent, yet it must be considered to be a
retrogression rather than an advance in organisation. The same remark
will apply to the whale tribe among mammals; to the blind amphibia and
insects of the great caverns; and among plants to the numerous cases in
which flowers, once specially adapted to be fertilised by insects, have
lost their gay corollas and their special adaptations, and have become
degraded into wind-fertilised forms. Such are our plantains, our meadow
burnet, and even, as some botanists maintain, our rushes, sedges, and
grasses. The causes which have led to this degeneration will be
discussed in a future chapter; but the facts are undisputed, and they
show us that although variation and the struggle for existence may lead,
on the whole, to a continued advance of organisation; yet they also lead
in many cases to a retrogression, when such retrogression may aid in the
preservation of any form under new conditions. They also lead to the
persistence, with slight modifications, of numerous lowly organised
forms which are suited to places which higher forms could not fully
occupy, or to conditions under which they could not exist. Such are the
ocean depths, the soil of the earth, the mud of rivers, deep caverns,
subterranean waters, etc.; and it is in such places as these, as well as
in some oceanic islands which competing higher forms have not been able
to reach, that we find many curious relics of an earlier world, which,
in the free air and sunlight and in the great continents, have long
since been driven out or exterminated by higher types.
_Summary of the first Five Chapters._
We have now passed in review, in more or less detail, the main facts on
which the theory of "the origin of species by means of natural
selection" is founded. In future chapters we shall have to deal mainly
with the application of the theory to explain the varied and complex
phenomena presented by the organic world; and, also, to discuss some of
the theories put forth by modern writers, either as being more
fundamental than that of Darwin or as supplementary to it. Before doing
this, however, it will be well briefly to summarise the facts and
arguments already set forth, because it is only by a clear comprehension
of these that the full importance of the theory can be appreciated and
its further applications understood.
The theory itself is exceedingly simple, and the facts on which it
rests--though excessively numerous individually, and coextensive with
the entire organic world--yet come under a few simple and easily
understood classes. These facts are,--first, the enormous powers of
increase in geometrical progression possessed by all organisms, and the
inevitable struggle for existence among them; and, in the second place,
the occurrence of much individual variation combined with the hereditary
transmission of such variations. From these two great classes of facts,
which are universal and indisputable, there necessarily arises, as
Darwin termed it, the "preservation of favoured races in the struggle
for life," the continuous action of which, under the ever-changing
conditions both of the inorganic and organic universe, necessarily leads
to the formation or development of new species.
But, although this general statement is complete and indisputable, yet
to see its applications under all the complex conditions that actually
occur in nature, it is necessary always to bear in mind the tremendous
power and universality of the agencies at work. We must never for an
instant lose sight of the fact of the enormously rapid increase of all
organisms, which has been illustrated by actual cases, given in our
second chapter, no less than by calculations of the results of unchecked
increase for a few years. Then, never forgetting that the animal and
plant population of any country is, on the whole, stationary, we must be
always trying to realise the ever-recurring destruction of the enormous
annual increase, and asking ourselves what determines, in each
individual case, the death of the many, the survival of the few. We must
think over all the causes of destruction to each organism,--to the seed,
the young shoot, the growing plant, the full-grown tree, or shrub, or
herb, and again the fruit and seed; and among animals, to the egg or
new-born young, to the youthful, and to the adults. Then, we must always
bear in mind that what goes on in the case of the individual or family
group we may observe or think of, goes on also among the millions and
scores of millions of individuals which are comprised in almost every
species; and must get rid of the idea that _chance_ determines which
shall live and which die. For, although in many individual cases death
may be due to chance rather than to any inferiority in those which die
first, yet we cannot possibly believe that this can be the case on the
large scale on which nature works. A plant, for instance, cannot be
increased unless there are suitable vacant places its seeds can grow in,
or stations where it can overcome other less vigorous and healthy
plants. The seeds of all plants, by their varied modes of dispersal, may
be said to be seeking out such places in which to grow; and we cannot
doubt that, in the long run, those individuals whose seeds are the most
numerous, have the greatest powers of dispersal, and the greatest vigour
of growth, will leave more descendants than the individuals of the same
species which are inferior in all these respects, although now and then
some seed of an inferior individual may _chance_ to be carried to a spot
where it can grow and survive. The same rule will apply to every period
of life and to every danger to which plants or animals are exposed. The
best organised, or the most healthy, or the most active, or the best
protected, or the most intelligent, will inevitably, in the long run,
gain an advantage over those which are inferior in these qualities; that
is, _the fittest will survive_, the fittest being, in each particular
case, those which are superior in the special qualities on which safety
depends. At one period of life, or to escape one kind of danger,
concealment may be necessary; at another time, to escape another danger,
swiftness; at another, intelligence or cunning; at another, the power to
endure rain or cold or hunger; and those which possess all these
faculties in the fullest perfection will generally survive.
Having fully grasped these facts in all their fulness and in their
endless and complex results, we have next to consider the phenomena of
variation, discussed in the third and fourth chapters; and it is here
that perhaps the greatest difficulty will be felt in appreciating the
full importance of the evidence as set forth. It has been so generally
the practice to speak of variation as something exceptional and
comparatively rare--as an abnormal deviation from the uniformity and
stability of the characters of a species--and so few even among
naturalists have ever compared, accurately, considerable numbers of
individuals, that the conception of variability as a general
characteristic of all dominant and widespread species, large in its
amount and affecting, not a few, but considerable masses of the
individuals which make up the species, will be to many entirely new.
Equally important is the fact that the variability extends to every
organ and every structure, external and internal; while perhaps most
important of all is the independent variability of these several parts,
each one varying without any constant or even usual dependence on, or
correlation with, other parts. No doubt there is some such correlation
in the differences that exist between species and species--more
developed wings usually accompanying smaller feet and _vice versâ_--but
this is, generally, a useful adaptation which has been brought about by
natural selection, and does not apply to the individual variability
which occurs within the species.
It is because these facts of variation are so important and so little
understood, that they have been discussed in what will seem to some
readers wearisome and unnecessary detail. Many naturalists, however,
will hold that even more evidence is required; and more, to almost any
amount, could easily have been given. The character and variety of that
already adduced will, however, I trust, convince most readers that the
facts are as stated; while they have been drawn from a sufficiently wide
area to indicate a general principle throughout nature.
If, now, we fully realise these facts of variation, along with those of
rapid multiplication and the struggle for existence, most of the
difficulties in the way of comprehending how species have originated
through natural selection will disappear. For whenever, through changes
of climate, or of altitude, or of the nature of the soil, or of the area
of the country, any species are exposed to new dangers, and have to
maintain themselves and provide for the safety of their offspring under
new and more arduous conditions, then, in the variability of all parts,
organs, and structures, no less than of habits and intelligence, we have
the means of producing modifications which will certainly bring the
species into harmony with its new conditions. And if we remember that
all such physical changes are slow and gradual in their operation, we
shall see that the amount of variation which we know occurs in every new
generation will be quite sufficient to enable modification and
adaptation to go on at the same rate. Mr. Darwin was rather inclined to
exaggerate the necessary slowness of the action of natural selection;
but with the knowledge we now possess of the great amount and range of
individual variation, there seems no difficulty in an amount of change,
quite equivalent to that which usually distinguishes allied species,
sometimes taking place in less than a century, should any rapid change
of conditions necessitate an equally rapid adaptation. This may often
have occurred, either to immigrants into a new land, or to residents
whose country has been cut off by subsidence from a larger and more
varied area over which they had formerly roamed. When no change of
conditions occurs, species may remain unchanged for very long periods,
and thus produce that appearance of stability of species which is even
now often adduced as an argument against evolution by natural selection,
but which is really quite in harmony with it.
On the principles, and by the light of the facts, now briefly
summarised, we have been able, in the present chapter, to indicate how
natural selection acts, how divergence of character is set up, how
adaptation to conditions at various periods of life has been effected,
how it is that low forms of life continue to exist, what kind of
circumstances are most favourable to the formation of new species, and,
lastly, to what extent the advance of organisation to higher types is
produced by natural selection. We will now pass on to consider some of
the more important objections and difficulties which have been advanced
by eminent naturalists.
FOOTNOTES:
[Footnote 37: _Origin of Species_, p. 71.]
[Footnote 38: Yarrell's _British Birds_, fourth edition, vol. iii. p.
77.]
[Footnote 39: _Origin of Species_, p. 89.]
[Footnote 40: _Nature_, vol. xxx. p. 30.]