character of the offspring of their domestic animals, yet any one
animal particularly useful to them, for any special purpose, would
be carefully preserved during famines and other accidents, to which
savages are so liable, and such choice animals would thus generally
leave more offspring than the inferior ones; so that in this case there
would be a kind of unconscious selection going on. We see the value set
on animals even by the barbarians of Tierra del Fuego, by their killing
and devouring their old women, in times of dearth, as of less value
than their dogs.
ADAPTATION OF ANIMALS TO THE FANCIES OF MAN.
[Page 28.]
On the view here given of the important part which selection by man has
played, it becomes at once obvious how it is that our domestic races
show adaptation in their structure or in their habits to man’s wants or
fancies. We can, I think, further understand the frequently abnormal
character of our domestic races, and likewise their differences being
so great in external characters, and relatively so slight in internal
parts or organs. Man can hardly select, or only with much difficulty,
any deviation of structure excepting such as is externally visible;
and, indeed, he rarely cares for what is internal. He can never act
by selection, excepting on variations which are first given to him in
some slight degree by nature. No man would ever try to make a fantail
till he saw a pigeon with a tail developed in some slight degree in
an unusual manner, or a pouter till he saw a pigeon with a crop of
somewhat unusual size; and the more abnormal or unusual any character
was when it first appeared, the more likely it would be to catch his
attention. But to use such an expression as trying to make a fantail
is, I have no doubt, in most cases, utterly incorrect. The man who
first selected a pigeon with a slightly larger tail, never dreamed what
the descendants of that pigeon would become through long-continued,
partly unconscious and partly methodical, selection. Perhaps the
parent-bird of all fantails had only fourteen tail-feathers somewhat
expanded, like the present Java fantail, or like individuals of other
and distinct breeds, in which as many as seventeen tail-feathers have
been counted. Perhaps the first pouter-pigeon did not inflate its crop
much more than the turbit now does the upper part of its œsophagus--a
habit which is disregarded by all fanciers, as it is not one of the
points of the breed.
DOUBTFUL SPECIES.
[Origin of
Species,
page 36.]
The forms which possess in some considerable degree the character of
species, but which are so closely similar to other forms, or are so
closely linked to them by intermediate gradations, that naturalists
do not like to rank them as distinct species, are in several respects
the most important for us. We have every reason to believe that many
of these doubtful and closely allied forms have permanently retained
their characters for a long time; for as long, as far as we know, as
have good and true species. Practically, when a naturalist can unite
by means of intermediate links any two forms, he treats the one as a
variety of the other; ranking the most common, but sometimes the one
first described, as the species, and the other as the variety. But
cases of great difficulty, which I will not here enumerate, sometimes
arise in deciding whether or not to rank one form as a variety of
another, even when they are closely connected by intermediate links;
nor will the commonly-assumed hybrid nature of the intermediate forms
always remove the difficulty. In very many cases, however, one form is
ranked as a variety of another, not because the intermediate links have
actually been found, but because analogy leads the observer to suppose
either that they do now somewhere exist, or may formerly have existed;
and here a wide door for the entry of doubt and conjecture is opened.
Hence, in determining whether a form should be ranked as a species or
a variety, the opinion of naturalists having sound judgment and wide
experience seems the only guide to follow. We must, however, in many
cases, decide by a majority of naturalists, for few well-marked and
well-known varieties can be named which have not been ranked as species
by at least some competent judges.
That varieties of this doubtful nature are far from uncommon can not
be disputed. Compare the several floras of Great Britain, of France,
or of the United States, drawn up by different botanists, and see what
a surprising number of forms have been ranked by one botanist as good
species, and by another as mere varieties. Mr. H. C. Watson, to whom
I lie under deep obligation for assistance of all kinds, has marked
for me one hundred and eighty-two British plants, which are generally
considered as varieties, but which have all been ranked by botanists
as species; and in making this list he has omitted many trifling
varieties, but which nevertheless have been ranked by some botanists as
species, and he has entirely omitted several highly polymorphic genera.
Under genera, including the most polymorphic forms, Mr. Babington gives
two hundred and fifty-one species, whereas Mr. Bentham gives only
one hundred and twelve--a difference of one hundred and thirty-nine
doubtful forms!
SPECIES AN ARBITRARY TERM.
[Page 41.]
Certainly no clear line of demarkation has as yet been drawn between
species and sub-species--that is, the forms which in the opinion
of some naturalists come very near to, but do not quite arrive at,
the rank of species; or, again, between sub-species and well-marked
varieties, or between lesser varieties and individual differences.
These differences blend into each other by an insensible series; and a
series impresses the mind with the idea of an actual passage.
Hence I look at individual differences, though of small interest to
the systematist, as of the highest importance for us, as being the
first steps toward such slight varieties as are barely thought worth
recording in works on natural history. And I look at varieties which
are in any degree more distinct and permanent as steps toward more
strongly-marked and permanent varieties; and at the latter, as leading
to sub-species, and then to species. The passage from one stage of
difference to another may, in many cases, be the simple result of the
nature of the organism, and of the different physical conditions to
which it has long been exposed; but with respect to the more important
and adaptive characters, the passage from one stage of difference to
another may be safely attributed to the cumulative action of natural
selection, hereafter to be explained, and to the effects of the
increased use or disuse of parts. A well-marked variety may therefore
be called an incipient species; but whether this belief is justifiable
must be judged by the weight of the various facts and considerations to
be given throughout this work.
It need not be supposed that all varieties or incipient species attain
the rank of species. They may become extinct, or they may endure as
varieties for very long periods, as has been shown to be the case by
Mr. Wollaston with the varieties of certain fossil land-shells in
Madeira, and with plants by Gaston de Saporta. If a variety were to
flourish so as to exceed in numbers the parent species, it would then
rank as the species, and the species as the variety; or it might come
to supplant and exterminate the parent species; or both might coexist,
and both rank as independent species. But we shall hereafter return to
this subject.
From these remarks it will be seen that I look at the term species
as one arbitrarily given, for the sake of convenience, to a set
of individuals closely resembling each other, and that it does
not essentially differ from the term variety, which is given to
less distinct and more fluctuating forms. The term variety, again,
in comparison with mere individual differences, is also applied
arbitrarily, for convenience’ sake.
THE TRUE PLAN OF CREATION.
[Origin of
Species,
page 425.]
When the views advanced by me in this volume, and by Mr. Wallace, or
when analogous views on the origin of species are generally admitted,
we can dimly foresee that there will be a considerable revolution in
natural history. Systematists will be able to pursue their labors as at
present; but they will not be incessantly haunted by the shadowy doubt
whether this or that form be a true species.
* * * * *
[Page 426.]
Hereafter we shall be compelled to acknowledge that the only
distinction between species and well-marked varieties is, that the
latter are known, or believed, to be connected at the present day by
intermediate gradations, whereas species were formerly thus connected.
Hence, without rejecting the consideration of the present existence
of intermediate gradations between any two forms, we shall be led
to weigh more carefully and to value higher the actual amount of
difference between them. It is quite possible that forms now generally
acknowledged to be merely varieties may hereafter be thought worthy of
specific names; and in this case scientific and common language will
come into accordance. In short, we shall have to treat species in the
same manner as those naturalists treat genera who admit that genera are
merely artificial combinations made for convenience. This may not be a
cheering prospect; but we shall at least be freed from the vain search
for the undiscovered and undiscoverable essence of the term species.
The other and more general departments of natural history will rise
greatly in interest. The terms used by naturalists, of affinity,
relationship, community of type, paternity, morphology, adaptive
characters, rudimentary and aborted organs, etc., will cease to be
metaphorical, and will have a plain signification. When we no longer
look at an organic being as a savage looks at a ship, as something
wholly beyond his comprehension; when we regard every production of
nature as one which has had a long history; when we contemplate every
complex structure and instinct as the summing up of many contrivances,
each useful to the possessor, in the same way as any great mechanical
invention is the summing up of the labor, the experience, the reason,
and even the blunders of numerous workmen; when we thus view each
organic being, how far more interesting--I speak from experience--does
the study of natural history become!
A grand and almost untrodden field of inquiry will be opened, on the
causes and laws of variation, on correlation, on the effects of use
and disuse, on the direct action of external conditions, and so forth.
The study of domestic productions will rise immensely in value. A new
variety raised by man will be a more important and interesting subject
for study than one more species added to the infinitude of already
recorded species. Our classifications will come to be, as far as they
can be so made, genealogies, and will then truly give what may be
called the plan of creation.
IV.
THE STRUGGLE FOR EXISTENCE.
[Origin of
Species,
page 50.]
A struggle for existence inevitably follows from the high rate at
which all organic beings tend to increase. Every being, which during
its natural lifetime produces several eggs or seeds, must suffer
destruction during some period of its life, and during some season or
occasional year, otherwise, on the principle of geometrical increase,
its numbers would quickly become so inordinately great that no country
could support the product. Hence, as more individuals are produced
than can possibly survive, there must in every case be a struggle for
existence, either one individual with another of the same species,
or with the individuals of distinct species, or with the physical
conditions of life. It is the doctrine of Malthus applied with
manifold force to the whole animal and vegetable kingdoms; for in this
case there can be no artificial increase of food, and no prudential
restraint from marriage. Although some species may be now increasing,
more or less rapidly, in numbers, all can not do so, for the world
would not hold them.
There is no exception to the rule that every organic being naturally
increases at so high a rate, that, if not destroyed, the earth would
soon be covered with the progeny of a single pair. Even slow-breeding
man has doubled in twenty-five years, and at this rate, in less than
a thousand years, there would literally not be standing-room for his
progeny. Linnæus has calculated that if an annual plant produced only
two seeds--and there is no plant so unproductive as this--and their
seedlings next year produced two, and so on, then in twenty years there
would be a million plants. The elephant is reckoned the slowest breeder
of all known animals, and I have taken some pains to estimate its
probable minimum rate of natural increase; it will be safest to assume
that it begins breeding when thirty years old, and goes on breeding
till ninety years old, bringing forth six young in the interval, and
surviving till one hundred years old; if this be so, after a period of
from seven hundred and forty to seven hundred and fifty years, there
would be nearly nineteen million elephants alive, descended from the
first pair.
DEATH INEVITABLE IN THE FIGHT FOR LIFE.
[Page 52.]
In a state of nature almost every full-grown plant annually produces
seed, and among animals there are very few which do not annually pair.
Hence we may confidently assert that all plants and animals are tending
to increase at a geometrical ratio, that all would rapidly stock every
station in which they could anyhow exist, and that this geometrical
tendency to increase must be checked by destruction at some period of
life. Our familiarity with the larger domestic animals tends, I think,
to mislead us: we see no great destruction falling on them, but we do
not keep in mind that thousands are annually slaughtered for food, and
that in a state of nature an equal number would have somehow to be
disposed of.
The only difference between organisms which annually produce eggs or
seeds by the thousand and those which produce extremely few is, that
the slow breeders would require a few more years to people, under
favorable conditions, a whole district, let it be ever so large. The
condor lays a couple of eggs and the ostrich a score, and yet in the
same country the condor may be the more numerous of the two; the Fulmar
petrel lays but one egg, yet it is believed to be the most numerous
bird in the world. One fly deposits hundreds of eggs, and another, like
the _Hippobosca_, a single one; but this difference does not determine
how many individuals of the two species can be supported in a district.
A large number of eggs is of some importance to those species which
depend on a fluctuating amount of food, for it allows them rapidly to
increase in number. But the real importance of a large number of eggs
or seeds is to make up for much destruction at some period of life;
and this period in the great majority of cases is an early one. If an
animal can in any way protect its own eggs or young, a small number may
be produced, and yet the average stock be fully kept up; but, if many
eggs or young are destroyed, many must be produced, or the species will
become extinct. It would suffice to keep up the full number of a tree,
which lived on an average for a thousand years, if a single seed were
produced once in a thousand years, supposing that this seed were never
destroyed, and could be insured to germinate in a fitting place. So
that, in all cases, the average number of any animal or plant depends
only indirectly on the number of its eggs or seeds.
In looking at Nature, it is most necessary to keep the foregoing
considerations always in mind--never to forget that every single
organic being may be said to be striving to the utmost to increase in
numbers; that each lives by a struggle at some period of its life;
that heavy destruction inevitably falls either on the young or old
during each generation or at recurrent intervals. Lighten any check,
mitigate the destruction ever so little, and the number of the species
will almost instantaneously increase to any amount.
“INEXPLICABLE ON THE THEORY OF CREATION.”
[Origin of
Species,
page 413.]
As each species tends by its geometrical rate of reproduction to
increase inordinately in number, and as the modified descendants of
each species will be enabled to increase by as much as they become more
diversified in habits and structure, so as to be able to seize on many
and widely different places in the economy of nature, there will be a
constant tendency in natural selection to preserve the most divergent
offspring of any one species. Hence, during a long-continued course
of modification, the slight differences characteristic of varieties
of the same species tend to be augmented into the greater differences
characteristic of the species of the same genus. New and improved
varieties will inevitably supplant and exterminate the older, less
improved, and intermediate varieties; and thus species are rendered
to a large extent defined and distinct objects. Dominant species
belonging to the larger groups within each class tend to give birth
to new and dominant forms; so that each large group tends to become
still larger, and at the same time more divergent in character. But,
as all groups can not thus go on increasing in size, for the world
would not hold them, the more dominant groups beat the less dominant.
This tendency in the large groups to go on increasing in size and
diverging in character, together with the inevitable contingency of
much extinction, explains the arrangement of all the forms of life in
groups subordinate to groups, all within a few great classes, which
has prevailed throughout all time. This grand fact of the grouping of
all organic beings under what is called the Natural System is utterly
inexplicable on the theory of creation.
OBSCURE CHECKS TO INCREASE.
[Origin of
Species,
page 53.]
The causes which check the natural tendency of each species to increase
are most obscure. Look at the most vigorous species; by as much as it
swarms in numbers, by so much will it tend to increase still further.
We know not exactly what the checks are even in a single instance. Nor
will this surprise any one who reflects how ignorant we are on this
head, even in regard to mankind, although so incomparably better known
than any other animal.
* * * * *
Eggs or very young animals seem generally to suffer most, but this is
not invariably the case. With plants there is a vast destruction of
seeds, but, from some observations which I have made it appears that
the seedlings suffer most from germinating in ground already thickly
stocked with other plants. Seedlings, also, are destroyed in vast
numbers by various enemies; for instance, on a piece of ground three
feet long and two wide, dug and cleared, and where there could be no
choking from other plants, I marked all the seedlings of our native
weeds as they came up, and out of 357 no less than 295 were destroyed,
chiefly by slugs and insects. If turf which has long been mown, and the
case would be the same with turf closely browsed by quadrupeds, be let
to grow, the more vigorous plants gradually kill the less vigorous,
though fully grown plants; thus out of twenty species growing on a
little plot of mown turf (three feet by four) nine species perished,
from the other species being allowed to grow up freely.
The amount of food for each species, of course, gives the extreme limit
to which each can increase; but very frequently it is not the obtaining
food, but the serving as prey to other animals, which determines the
average number of a species. Thus, there seems to be little doubt that
the stock of partridges, grouse, and hares on any large estate depends
chiefly on the destruction of vermin. If not one head of game were shot
during the next twenty years in England, and, at the same time, if no
vermin were destroyed, there would, in all probability, be less game
than at present, although hundreds of thousands of game animals are now
annually shot. On the other hand, in some cases, as with the elephant,
none are destroyed by beasts of prey; for even the tiger in India most
rarely dares to attack a young elephant protected by its dam.
CLIMATE AS A CHECK TO INCREASE.
[Page 54.]
Climate plays an important part in determining the average numbers of
a species, and periodical seasons of extreme cold or drought seem to
be the most effective of all checks. I estimated (chiefly from the
greatly reduced numbers of nests in the spring) that the winter of
1854–’55 destroyed four fifths of the birds in my own grounds; and this
is a tremendous destruction, when we remember that ten per cent is an
extraordinarily severe mortality from epidemics with man. The action of
climate seems at first sight to be quite independent of the struggle
for existence; but, in so far as climate chiefly acts in reducing
food, it brings on the most severe struggle between the individuals,
whether of the same or of distinct species, which subsist on the same
kind of food. Even when climate--for instance, extreme cold--acts
directly, it will be the least vigorous individuals, or those which
have got least food through the advancing winter, which will suffer
most. When we travel from south to north, or from a damp region to a
dry, we invariably see some species gradually getting rarer and rarer,
and finally disappearing; and, the change of climate being conspicuous,
we are tempted to attribute the whole effect to its direct action. But
this is a false view: we forget that each species, even where it most
abounds, is constantly suffering enormous destruction at some period
of its life, from enemies or from competitors for the same place and
food; and, if these enemies or competitors be in the least degree
favored by any slight change of climate, they will increase in numbers;
and, as each area is already fully stocked with inhabitants, the other
species must decrease. When we travel southward and see a species
decreasing in numbers, we may feel sure that the cause lies quite as
much in other species being favored as in this one being hurt. So it
is when we travel northward, but in a somewhat lesser degree, for the
number of species of all kinds, and therefore of competitors, decreases
northward; hence, in going northward, or in ascending a mountain, we
far oftener meet with stunted forms, due to the _directly_ injurious
action of climate, than we do in proceeding southward or in descending
a mountain. When we reach the Arctic regions, or snow-capped summits,
or absolute deserts, the struggle for life is almost exclusively with
the elements.
INFLUENCE OF INSECTS IN THE STRUGGLE FOR EXISTENCE.
[Page 56.]