preserved, whether in their egg, or larval, or mature state, which are
best adapted to the place they fill in nature, there is a second agency
at work in most unisexual animals, tending to produce the same effect,
namely, the struggle of the males for the females. These struggles are
generally decided by the law of battle, but in the case of birds,
apparently, by the charms of their song, by their beauty or their power
of courtship, as in the dancing rock-thrush of Guiana. The most vigorous
and healthy males, implying perfect adaptation, must generally gain the
victory in their contests. This kind of selection, however, is less
rigorous than the other; it does not require the death of the less
successful, but gives to them fewer descendants. The struggle falls,
moreover, at a time of year when food is generally abundant, and perhaps
the effect chiefly produced would be the modification of the secondary
sexual characters, which are not related to the power of obtaining food,
or to defence from enemies, but to fighting with or rivalling other
males. The result of this struggle amongst the males may be compared in
some respects to that produced by those agriculturists who pay less
attention to the careful selection of all their young animals, and more
to the occasional use of a choice mate.
II. _Abstract of a Letter from_ C. DARWIN, Esq., _to_ Prof. ASA GRAY,
_Boston, U.S., dated Down, September 5th, 1857._
1. It is wonderful what the principle of selection by man, that is the
picking out of individuals with any desired quality, and breeding from
them, and again picking out, can do. Even breeders have been astounded
at their own results. They can act on differences inappreciable to an
uneducated eye. Selection has been _methodically_ followed in _Europe_
for only the last half century; but it was occasionally, and even in
some degree methodically, followed in the most ancient times. There must
have been also a kind of unconscious selection from a remote period,
namely in the preservation of the individual animals (without any
thought of their offspring) most useful to each race of man in his
particular circumstances. The "roguing," as nurserymen call the
destroying of varieties which depart from their type, is a kind of
selection. I am convinced that intentional and occasional selection has
been the main agent in the production of our domestic races; but however
this may be, its great power of modification has been indisputably shown
in later times. Selection acts only by the accumulation of slight or
greater variations, caused by external conditions, or by the mere fact
that in generation the child is not absolutely similar to its parent.
Man, by this power of accumulating variations, adapts living beings to
his wants--may be said to make the wool of one sheep good for carpets,
of another for cloth, &c.
2. Now suppose there were a being who did not judge by mere external
appearances, but who could study the whole internal organization, who
was never capricious, and should go on selecting for one object during
millions of generations; who will say what he might not effect? In
nature we have some _slight_ variation occasionally in all parts; and I
think it can be shown that changed conditions of existence is the main
cause of the child not exactly resembling its parents; and in nature
geology shows us what changes have taken place, and are taking place. We
have almost unlimited time; no one but a practical geologist can fully
appreciate this. Think of the Glacial period, during the whole of which
the same species at least of shells have existed; there must have been
during this period millions on millions of generations.
3. I think it can be shown that there is such an unerring power at work
in _Natural Selection_ (the title of my book), which selects exclusively
for the good of each organic being. The elder De Candolle, W. Herbert,
and Lyell have written excellently on the struggle for life; but even
they have not written strongly enough. Reflect that every being (even
the elephant) breeds at such a rate, that in a few years, or at most a
few centuries, the surface of the earth would not hold the progeny of
one pair. I have found it hard constantly to bear in mind that the
increase of every single species is checked during some part of its
life, or during some shortly recurrent generation. Only a few of those
annually born can live to propagate their kind. What a trifling
difference must often determine which shall survive, and which perish!
4. Now take the case of a country undergoing some change. This will tend
to cause some of its inhabitants to vary slightly--not but that I
believe most beings vary at all times enough for selection to act on
them. Some of its inhabitants will be exterminated; and the remainder
will be exposed to the mutual action of a different set of inhabitants,
which I believe to be far more important to the life of each being than
mere climate. Considering the infinitely various methods which living
beings follow to obtain food by struggling with other organisms, to
escape danger at various times of life, to have their eggs or seeds
disseminated, &c. &c., I cannot doubt that during millions of
generations individuals of a species will be occasionally born with some
slight variation, profitable to some part of their economy. Such
individuals will have a better chance of surviving, and of propagating
their new and slightly different structure; and the modification may be
slowly increased by the accumulative action of natural selection to any
profitable extent. The variety thus formed will either coexist with, or,
more commonly, will exterminate its parent form. An organic being, like
the woodpecker or misseltoe, may thus come to be adapted to a score of
contingences--natural selection accumulating those slight variations in
all parts of its structure, which are in any way useful to it during any
part of its life.
5. Multiform difficulties will occur to every one, with respect to this
theory. Many can, I think, be satisfactorily answered. _Natura non facit
saltum_ answers some of the most obvious. The slowness of the change,
and only a very few individuals undergoing change at any one time,
answers others. The extreme imperfection of our geological records
answers others.
6. Another principle, which may be called the principle of divergence,
plays, I believe, an important part in the origin of species. The same
spot will support more life if occupied by very diverse forms. We see
this in the many generic forms in a square yard of turf, and in the
plants or insects on any little uniform islet, belonging almost
invariably to as many genera and families as species. We can understand
the meaning of this fact amongst the higher animals, whose habits we
understand. We know that it has been experimentally shown that a plot of
land will yield a greater weight if sown with several species and genera
of grasses, than if sown with only two or three species. Now, every
organic being, by propagating so rapidly, may be said to be striving its
utmost to increase in numbers. So it will be with the offspring of any
species after it has become diversified into varieties, or subspecies,
or true species. And it follows, I think, from the foregoing facts, that
the varying offspring of each species will try (only few will succeed)
to seize on as many and as diverse places in the economy of nature as
possible. Each new variety or species, when formed, will generally take
the place of, and thus exterminate its less well-fitted parent. This I
believe to be the origin of the classification and affinities of organic
beings at all times; for organic beings always _seem_ to branch and
sub-branch like the limbs of a tree from a common trunk, the flourishing
and diverging twigs destroying the less vigorous--the dead and lost
branches rudely representing extinct genera and families.
This sketch is _most_ imperfect; but in so short a space I cannot make
it better. Your imagination must fill up very wide blanks.
C. DARWIN.
III. _On the Tendency of Varieties to depart indefinitely from the
Original Type._ By ALFRED RUSSELL WALLACE.
One of the strongest arguments which have been adduced to prove the
original and permanent distinctness of species is, that _varieties_
produced in a state of domesticity are more or less unstable, and often
have a tendency, if left to themselves, to return to the normal form of
the parent species; and this instability is considered to be a
distinctive peculiarity of all varieties, even of those occurring among
wild animals in a state of nature, and to constitute a provision for
preserving unchanged the originally created distinct species.
In the absence or scarcity of facts and observations as to _varieties_
occurring among wild animals, this argument has had great weight with
naturalists, and has led to a very general and somewhat prejudiced
belief in the stability of species. Equally general, however, is the
belief in what are called "permanent or true varieties,"--races of
animals which continually propagate their like, but which differ so
slightly (although constantly) from some other race, that the one is
considered to be a _variety_ of the other. Which is the _variety_ and
which the original _species_, there is generally no means of
determining, except in those rare cases in which the one race has been
known to produce an offspring unlike itself and resembling the other.
This, however, would seem quite incompatible with the "permanent
invariability of species," but the difficulty is overcome by assuming
that such varieties have strict limits, and can never again vary further
from the original type, although they may return to it, which, from the
analogy of the domesticated animals, is considered to be highly
probable, if not certainly proved.
It will be observed that this argument rests entirely on the assumption,
that _varieties_ occurring in a state of nature are in all respects
analogous to or even identical with those of domestic animals, and are
governed by the same laws as regards their permanence or further
variation. But it is the object of the present paper to show that this
assumption is altogether false, that there is a general principle in
nature which will cause many _varieties_ to survive the parent species,
and to give rise to successive variations departing further and further
from the original type, and which also produces, in domesticated
animals, the tendency of varieties to return to the parent form.
The life of wild animals is a struggle for existence. The full exertion
of all their faculties and all their energies is required to preserve
their own existence and provide for that of their infant offspring. The
possibility of procuring food during the least favourable seasons, and
of escaping the attacks of their most dangerous enemies, are the primary
conditions which determine the existence both of individuals and of
entire species. These conditions will also determine the population of a
species; and by a careful consideration of all the circumstances we may
be enabled to comprehend, and in some degree to explain, what at first
sight appears so inexplicable--the excessive abundance of some species,
while others closely allied to them are very rare.
The general proportion that must obtain between certain groups of
animals is readily seen. Large animals cannot be so abundant as small
ones; the carnivora must be less numerous than the herbivora; eagles and
lions can never be so plentiful as pigeons and antelopes; the wild asses
of the Tartarian deserts cannot equal in numbers the horses of the more
luxuriant prairies and pampas of America. The greater or less fecundity
of an animal is often considered to be one of the chief causes of its
abundance or scarcity; but a consideration of the facts will show us
that it really has little or nothing to do with the matter. Even the
least prolific of animals would increase rapidly if unchecked, whereas
it is evident that the animal population of the globe must be
stationary, or perhaps, through the influence of man, decreasing.
Fluctuations there may be; but permanent increase, except in restricted
localities, is almost impossible. For example, our own observation must
convince us that birds do not go on increasing every year in a
geometrical ratio, as they would do, were there not some powerful check
to their natural increase. Very few birds produce less than two young
ones each year, while many have six, eight, or ten; four will certainly
be below the average; and if we suppose that each pair produce young
only four times in their life, that will also be below the average,
supposing them not to die either by violence or want of food. Yet at
this rate how tremendous would be the increase in a few years from a
single pair! A simple calculation will show that in fifteen years each
pair of birds would have increased to nearly ten millions! whereas we
have no reason to believe that the number of the birds of any country
increases at all in fifteen or in one hundred and fifty years. With such
powers of increase the population must have reached its limits, and have
become stationary, in a very low years after the origin of each species.
It is evident, therefore, that each year an immense number of birds must
perish--as many in fact as are born; and as on the lowest calculation
the progeny are each year twice as numerous as their parents, it follows
that, whatever be the average number of individuals existing in any
given country, _twice that number must perish annually_,--a striking
result, but one which seems at least highly probable, and is perhaps
under rather than over the truth. It would therefore appear that, as far
as the continuance of the species and the keeping up the average number
of individuals are concerned, large broods are superfluous. On the
average all above _one_ become food for hawks and kites, wild cats and
weasels, or perish of cold and hunger as winter comes on. This is
strikingly proved by the case of particular species; for we find that
their abundance in individuals bears no relation whatever to their
fertility in producing offspring. Perhaps the most remarkable instance
of an immense bird population is that of the passenger pigeon of the
United States, which lays only one, or at most two eggs, and is said to
rear generally but one young one. Why is this bird so extraordinarily
abundant, while others producing two or three times as many young are
much less plentiful? The explanation is not difficult. The food most
congenial to this species, and on which it thrives best, is abundantly
distributed over a very extensive region, offering such differences of
soil and climate, that in one part or another of the area the supply
never fails. The bird is capable of a very rapid and long-continued
flight, so that it can pass without fatigue over the whole of the
district it inhabits, and as soon as the supply of food begins to fail
in one place is able to discover a fresh feeding-ground. This example
strikingly shows us that the procuring a constant supply of wholesome
food is almost the sole condition requisite for ensuring the rapid
increase of a given species, since neither the limited fecundity, nor
the unrestrained attacks of birds of prey and of man are here sufficient
to check it. In no other birds are these peculiar circumstances so
strikingly combined. Either their food is more liable to failure, or
they have not sufficient power of wing to search for it over an
extensive area, or during some season of the year it becomes very
scarce, and less wholesome substitutes have to be found; and thus,
though more fertile in offspring, they can never increase beyond the
supply of food in the least favourable seasons. Many birds can only
exist by migrating, when their food becomes scarce, to regions
possessing a milder, or at least a different climate, though, as these
migrating birds are seldom excessively abundant, it is evident that the
countries they visit are still deficient in a constant and abundant
supply of wholesome food. Those whose organization does not permit them
to migrate when their food becomes periodically scarce, can never attain
a large population. This is probably the reason why woodpeckers are
scarce with us, while in the tropics they are among the most abundant of
solitary birds. Thus the house sparrow is more abundant than the
redbreast, because its food is more constant and plentiful,--seeds of
grasses being preserved during the winter, and our farm-yards and
stubble-fields furnishing an almost inexhaustible supply. Why, as a
general rule, are aquatic, and especially sea birds, very numerous in
individuals? Not because they are more prolific than others, generally
the contrary; but because their food never fails, the sea-shores and
river-banks daily swarming with a fresh supply of small mollusca and
crustacea. Exactly the same laws will apply to mammals. Wild cats are
prolific and have few enemies; why then are they never as abundant as
rabbits? The only intelligible answer is, that their supply of food is
more precarious. It appears evident, therefore, that so long as a
country remains physically unchanged, the numbers of its animal
population cannot materially increase. If one species does so, some
others requiring the same kind of food must diminish in proportion. The
numbers that die annually must be immense; and as the individual
existence of each animal depends upon itself, those that die must be the
weakest--the very young, the aged, and the diseased,--while those that
prolong their existence can only be the most perfect in health and
vigour--those who are best able to obtain food regularly, and avoid
their numerous enemies. It is, as we commenced by remarking, "a struggle
for existence," in which the weakest and least perfectly organized must
always succumb.
Now it is clear that what takes place among the individuals of a species
must also occur among the several allied species of a group,--viz. that
those which are best adapted to obtain a regular supply of food, and to
defend themselves against the attacks of their enemies and the
vicissitudes of the seasons, must necessarily obtain and preserve a
superiority in population; while those species which from some defect of
power or organization are the least capable of counteracting the
vicissitudes of food, supply, &c., must diminish in numbers, and, in
extreme cases, become altogether extinct. Between these extremes the
species will present various degrees of capacity for ensuring the means
of preserving life; and it is thus we account for the abundance or
rarity of species. Our ignorance will generally prevent us from
accurately tracing the effects to their causes; but could we become
perfectly acquainted with the organization and habits of the various
species of animals, and could we measure the capacity of each for
performing the different acts necessary to its safety and existence
under all the varying circumstances by which it is surrounded, we might
be able even to calculate the proportionate abundance of individuals
which is the necessary result.
If now we have succeeded in establishing these two points--1st, _that
the animal population of a country is generally stationary, being kept
down by a periodical deficiency of food, and other checks_; and, 2nd,
_that the comparative abundance or scarcity of the individuals of the
several species is entirely due to their organization and resulting
habits, which, rendering it more difficult to procure a regular supply
of food and to provide for their personal safety in some cases than in
others, can only be balanced by a difference in the population which
have to exist in a given area_--we shall be in a condition to proceed to
the consideration of _varieties_, to which the preceding remarks have a
direct and very important application.
Most or perhaps all the variations from the typical form of a species
must have some definite effect, however slight, on the habits or
capacities of the individuals. Even a change of colour might, by
rendering them more or less distinguishable, affect their safety; a
greater or less development of hair might modify their habits. More
important changes, such as an increase in the power or dimensions of the
limbs or any of the external organs, would more or less affect their
mode of procuring food or the range of country which they inhabit. It
is also evident that most changes would affect, either favourably or
adversely, the powers of prolonging existence. An antelope with shorter
or weaker legs must necessarily suffer more from the attacks of the
feline carnivora; the passenger pigeon with less powerful wings would
sooner or later be affected in its powers of procuring a regular supply
of food; and in both cases the result must necessarily be a diminution
of the population of the modified species. If, on the other hand, any
species should produce a variety having slightly increased powers of
preserving existence, that variety must inevitably in time acquire a
superiority in numbers. These results must follow as surely as old age,
intemperance, or scarcity of food produce an increased mortality. In
both cases there may be many individual exceptions; but on the average
the rule will invariably be found to hold good. All varieties will
therefore fall into two classes--those which under the same conditions
would never reach the population of the parent species, and those which
would in time obtain and keep a numerical superiority. Now, let some
alteration of physical conditions occur in the district--a long period
of drought, a destruction of vegetation by locusts, the irruption of
some new carnivorous animal seeking "pastures new"--any change in fact
tending to render existence more difficult to the species in question,
and tasking its utmost powers to avoid complete extermination; it is
evident that, of all the individuals composing the species, those
forming the least numerous and most feebly organized variety would
suffer first, and, were the pressure severe, must soon become extinct.
The same causes continuing in action, the parent species would next
suffer, would gradually diminish in numbers, and with a recurrence of
similar unfavourable conditions might also become extinct. The superior
variety would then alone remain, and on a return to favourable
circumstances would rapidly increase in numbers and occupy the place of
the extinct species and variety.