Darwinism (1889) by Alfred Russel Wallace
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Alfred Russel Wallace >> Darwinism (1889)
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In the vegetable kingdom, most of the cereals--wheat, barley, etc.--are
unknown as truly wild plants; and the same is the case with many
vegetables, for De Candolle states that out of 157 useful cultivated
plants thirty-two are quite unknown in a wild state, and that forty more
are of doubtful origin. It is not improbable that most of these do exist
wild, but they have been so profoundly changed by thousands of years of
cultivation as to be quite unrecognisable. The peach is unknown in a
wild state, unless it is derived from the common almond, on which point
there is much difference of opinion among botanists and horticulturists.
The immense antiquity of most of our cultivated plants sufficiently
explains the apparent absence of such useful productions in Australia
and the Cape of Good Hope, notwithstanding that they both possess an
exceedingly rich and varied flora. These countries having been, until a
comparatively recent period, inhabited only by uncivilised men, neither
cultivation nor selection has been carried on for a sufficiently long
time. In North America, however, where there was evidently a very
ancient if low form of civilisation, as indicated by the remarkable
mounds, earthworks, and other prehistoric remains, maize was cultivated,
though it was probably derived from Peru; and the ancient civilisation
of that country and of Mexico has given rise to no fewer than
thirty-three useful cultivated plants.
_Conditions favourable to the production of Variations._
In order that plants and animals may be improved and modified to any
considerable extent, it is of course essential that suitable variations
should occur with tolerable frequency. There seem to be three conditions
which are especially favourable to the production of variations: (1)
That the particular species or variety should be kept in very large
numbers; (2) that it should be spread over a wide area and thus
subjected to a considerable diversity of physical conditions; and (3)
that it should be occasionally crossed with some distinct but closely
allied race. The first of these conditions is perhaps the most
important, the chance of variations of any particular kind being
increased in proportion to the quantity of the original stock and of its
annual offspring. It has been remarked that only those breeders who keep
large flocks can effect much improvement; and it is for the same reason
that pigeons and fowls, which can be so easily and rapidly increased,
and which have been kept in such large numbers by so great a number of
persons, have produced such strange and numerous varieties. In like
manner, nurserymen who grow fruit and flowers in large quantities have a
great advantage over private amateurs in the production of new
varieties.
Although I believe, for reasons which will be given further on, that
some amount of variability is a constant and necessary property of all
organisms, yet there appears to be good evidence to show that changed
conditions of life tend to increase it, both by a direct action on the
organisation and by indirectly affecting the reproductive system. Hence
the extension of civilisation, by favouring domestication under altered
conditions, facilitates the process of modification. Yet this change
does not seem to be an essential condition, for nowhere has the
production of extreme varieties of plants and flowers been carried
farther than in Japan, where careful selection continued for many
generations must have been the chief factor. The effect of occasional
crosses often results in a great amount of variation, but it also leads
to instability of character, and is therefore very little employed in
the production of fixed and well-marked races. For this purpose, in
fact, it has to be carefully avoided, as it is only by isolation and
pure breeding that any specially desired qualities can be increased by
selection. It is for this reason that among savage peoples, whose
animals run half wild, little improvement takes place; and the
difficulty of isolation also explains why distinct and pure breeds of
cats are so rarely met with. The wide distribution of useful animals and
plants from a very remote epoch has, no doubt, been a powerful cause of
modification, because the particular breed first introduced into each
country has often been kept pure for many years, and has also been
subjected to slight differences of conditions. It will also usually have
been selected for a somewhat different purpose in each locality, and
thus very distinct races would soon originate.
The important physiological effects of crossing breeds or strains, and
the part this plays in the economy of nature, will be explained in a
future chapter.
_Concluding Remarks._
The examples of variation now adduced--and these might have been almost
indefinitely increased--will suffice to show that there is hardly an
organ or a quality in plants or animals which has not been observed to
vary; and further, that whenever any of these variations have been
useful to man he has been able to increase them to a marvellous extent
by the simple process of always preserving the best varieties to breed
from. Along with these larger variations others of smaller amount
occasionally appear, sometimes in external, sometimes in internal
characters, the very bones of the skeleton often changing slightly in
form, size, or number; but as these secondary characters have been of no
use to man, and have not been specially selected by him, they have,
usually, not been developed to any great amount except when they have
been closely dependent on those external characters which he has largely
modified.
As man has considered only utility to himself, or the satisfaction of
his love of beauty, of novelty, or merely of something strange or
amusing, the variations he has thus produced have something of the
character of monstrosities. Not only are they often of no use to the
animals or plants themselves, but they are not unfrequently injurious to
them. In the Tumbler pigeons, for instance, the habit of tumbling is
sometimes so excessive as to injure or kill the bird; and many of our
highly-bred animals have such delicate constitutions that they are very
liable to disease, while their extreme peculiarities of form or
structure would often render them quite unfit to live in a wild state.
In plants, many of our double flowers, and some fruits, have lost the
power of producing seed, and the race can thus be continued only by
means of cuttings or grafts. This peculiar character of domestic
productions distinguishes them broadly from wild species and varieties,
which, as will be seen by and by, are necessarily adapted in every part
of their organisation to the conditions under which they have to live.
Their importance for our present inquiry depends on their demonstrating
the occurrence of incessant slight variations in all parts of an
organism, with the transmission to the offspring of the special
characteristics of the parents; and also, that all such slight
variations are capable of being accumulated by selection till they
present very large and important divergencies from the ancestral stock.
We thus see, that the evidence as to variation afforded by animals and
plants under domestication strikingly accords with that which we have
proved to exist in a state of nature. And it is not at all surprising
that it should be so, since all the species were in a state of nature
when first domesticated or cultivated by man, and whatever variations
occur must be due to purely natural causes. Moreover, on comparing the
variations which occur in any one generation of domesticated animals
with those which we know to occur in wild animals, we find no evidence
of greater individual variation in the former than in the latter. The
results of man's selection are more striking to us because we have
always considered the varieties of each domestic animal to be
essentially identical, while those which we observe in a wild state are
held to be essentially diverse. The greyhound and the spaniel seem
wonderful, as varieties of one animal produced by man's selection; while
we think little of the diversities of the fox and the wolf, or the horse
and the zebra, because we have been accustomed to look upon them as
radically distinct animals, not as the results of nature's selection of
the varieties of a common ancestor.
FOOTNOTES:
[Footnote 31: Darwin, _Animals and Plants under Domestication_, vol. i.
p. 322.]
[Footnote 32: These facts are taken from Darwin's _Domesticated Animals
and Cultivated Plants_, vol. i. pp. 359, 360, 392-401; vol. ii. pp. 231,
275, 330.]
[Footnote 33: See Darwin's _Animals and Plants under Domestication_,
vol. i. pp. 40-42.]
[Footnote 34: Mr. Brent in _Journal of Horticulture_, 1861, p. 76;
quoted by Darwin, _Animals and Plants under Domestication_, vol. i. p.
151.]
[Footnote 35: This account of domestic pigeons is greatly condensed from
Mr. Darwin's work already referred to.]
[Footnote 36: _Animals and Plants under Domestication_, vol. ii. pp.
307-311.]
CHAPTER V
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.
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