Letters to the Royal Society, Charles Darwin & Alfred R. Wallace, 1858


ON THE TENDENCY OF SPECIES TO FORM VARIETIES AND ON THE PERPETUATION OF VARIETIES AND SPECIES BY NATURAL MEANS OF SELECTION.

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By CHARLES DARWIN, Esq., F.R.S., F.L.S., & F.G.S.,
and ALFRED WALLACE, Esq.
Communicated by Sir CHARLES LYELL, F.R.S., F.L.S.,
and J.D. HOOKER, Esq., M.D., V.P.R.S., F.L.S., &c.
[Read July 1st, 1858.] London, June 30th, 1858.
MY DEAR SIR, -- The accompanying papers, which we have the honour of
communicating to the Linnean Society, and which all relate to the same subject,
viz. the Laws which affect the Production of Varieties, Races, and Species,
contain the results of the investigations of two indefatigable naturalists, Mr.
Charles Darwin and Mr. Alfred Wallace.
These gentlemen having, independently and unknown to one another,
conceived the same very ingenious theory to account for the appearance and
perpetuation of varieties and of specific forms on our planet, may both fairly
claim the merit of being original thinkers in this important line of inquiry;
but neither of them having published his views, though Mr. Darwin has for many
years past been repeatedly urged by us to do so, and both authors having now
unreservedly placed their papers in our hands, we think it would best promote
the interests of science that a selection from them should be laid before the
Linnean Society.
Taken in the order of their dates, they consist of:--
1. Extracts from a MS. work on Species [*], by Mr. Darwin, which was
sketched in 1839, and copied in 1844, when the copy was read by Dr. Hooker, and
its contents afterwards communicated to Sir Charles Lyell. The first Part is
devoted to "The Variation of Organic Beings under Domestication and in their
Natural State;" and the second chapter of that Part, from which we propose to
read to the Society the extracts referred to, is headed, "On the Variation of
Organic Beings in a state of Nature; on the Natural Means of Selection; on the
Comparison of Domestic Races and true Species."
2. An abstract of a private letter addressed to Professor Asa Gray, of
Boston, U.S., in October 1857, by Mr. Darwin, in which he repeats his views,
and which shows that these remained unaltered from 1839 to 1857.
3. An Essay by Mr. Wallace, entitled "On the Tendency of Varieties to
depart indefinitely from the Original Type." This was written at Ternate in
February 1858, for the perusal of his friend and correspondent Mr. Darwin, and
sent to him with the expressed wish that it should be forwarded to Sir Charles
Lyell, if Mr. Darwin thought it sufficiently novel and interesting. So highly
did Mr. Darwin appreciate the value of the views therein set forth, that he
proposed, in a letter to Sir Charles Lyell, to obtain Mr. Wallace's consent to
allow the Essay to be published as soon as possible. Of this step we highly
approved, provided Mr. Darwin did not withhold from the public, as he was
strongly inclined to do (in favour of Mr. Wallace), the memoir which he had
himself written on the same subject, and which, as before stated, one of us had
perused in 1844, and the contents of which we had both of us been privy to for
many years. On representing this to Mr. Darwin, he gave us permission to make
what use we thought proper of his memoir, &c.; and in adopting our present
course, of presenting it to the Linnean Society, we have explained to him that
we are not solely considering the relative claims to priority of himself and
his friend, but the interests of science generally; for we feel it to be
desirable that views founded on a wide deduction from facts, and matured by
years of reflection, should constitute at once a goal from which others may
start, and that, while the scientific world is waiting for the appearance of
Mr. Darwin's complete work, some of the leading results of his labours, as well
as those of his able correspondent, should together be laid before the public.
We have the honour to be yours very obediently,
CHARLES LYELL.
JOS. D. HOOKER.
J. J. Bennett, Esq.,
Secretary of the Linnean Society.

I. Extract from an unpublished Work on Species, by C. DARWIN, Esq., consisting of a portion of a Chapter entitled, "On the Variation of Organic Beings in a state of Nature; on the Natural Means of Selection; on the Comparison of Domestic Races and true S


De Candolle, in an eloquent passage, has declared that all nature is at
war, one organism with another, or with external nature. Seeing the contented
face of nature, this may at first well be doubted; but reflection will
inevitably prove it to be true. The war, however, is not constant, but
recurrent in a slight degree at short periods, and more severely at occasional
more distant periods; and hence its effects are easily overlooked. It is the
doctrine of Malthus applied in most cases with tenfold force. As in every
climate there are seasons, for each of its inhabitants, of greater and less
abundance, so all annually breed; and the moral restraint which in some small
degree checks the increase of mankind is entirely lost. Even slow-breeding
mankind has doubled in twenty-five years; and if he could increase his food
with greater ease, he would double in less time. But for animals without
artificial means, the amount of food for each species must, on an average, be
constant, whereas the increase of all organisms tends to be geometrical, and in
a vast majority of cases at an enormous ratio. Suppose in a certain spot there
are eight pairs of birds, and that only four pairs of them annually (including
double hatches) rear only four young, and that these go on rearing their young
at the same rate, then at the end of seven years (a short life, excluding
violent deaths, for any bird) there will be 2048 birds, instead of the original
sixteen. As this increase is quite impossible, we must conclude either that
birds do not rear nearly half their young, or that the average life of a bird
is, from accident, not nearly seven years. Both checks probably concur. The
same kind of calculation applied to all plants and animals affords results more
or less striking, but in very few instances more striking than in man.
Many practical illustrations of this rapid tendency to increase are on
record, among which, during peculiar seasons, are the extraordinary numbers of
certain animals; for instance, during the years 1826 to 1828, in La Plata, when
from drought some millions of cattle perished, the whole country actually
swarmed with mice. Now I think it cannot be doubted that during the
breeding-season all the mice (with the exception of a few males or females in
excess) ordinarily pair, and therefore that this astounding increase during
three years must be attributed to a greater number than usual surviving the
first year, and then breeding, and so on till the third year, when their
numbers were brought down to their usual limits on the return of wet weather.
Where man has introduced plants and animals into a new and favourable country,
there are many accounts in how surprisingly few years the whole country has
become stocked with them. This increase would necessarily stop as soon as the
country was fully stocked; and yet we have every reason to believe, from what
is known of wild animals, that all would pair in the spring. In the majority
of cases it is most difficult to imagine where the checks fall--though
generally, no doubt, on the seeds, eggs, and young; but when we remember how
impossible, even in mankind (so much better known than any other animal), it is
to infer from repeated casual observations what the average duration of life
is, or to discover the different percentage of deaths to births in different
countries, we ought to feel no surprise at our being unable to discover where
the check falls in any animal or plant. It should always be remembered, that
in most cases the checks are recurrent yearly in a small, regular degree, and
in an extreme degree during unusually cold, hot, dry, or wet years, according
to the constitution of the being in question. Lighten any check in the least
degree, and the geometrical powers of increase in every organism will almost
instantly increase the average number of the favoured species. Nature may be
compared to a surface on which rest ten thousand sharp wedges touching each
other and driven inwards by incessant blows. Fully to realize these views much
reflection is requisite. Malthus on man should be studied; and all such cases
as those of the mice in La Plata, of the cattle and horses when first turned
out in South America, of the birds by our calculation, &c., should be well
considered. Reflect on the enormous multiplying power inherent and annually in
action in all animals; reflect on the countless seeds scattered by a hundred
ingenious contrivances, year after year, over the whole face of the land; and
yet we have every reason to suppose that the average percentage of each of the
inhabitants of a country usually remains constant. Finally, let it be borne in
mind that this average number of individuals (the external conditions remaining
the same) in each country is kept up by recurrent struggles against other
species or against external nature (as on the borders of the Arctic regions,
where the cold checks life), and that ordinarily each individual of every
species holds its place, either by its own struggle and capacity of acquiring
nourishment in some period of its life, from the egg upwards; or by the
struggle of its parents (in short-lived organisms, when the main check occurs
at longer intervals) with other individuals of the same or different species.
But let the external conditions of a country alter. If in a small degree,
the relative proportions of the inhabitants will in most cases simply be
slightly changed; but let the number of inhabitants be small, as on an island,
and free access to it from other countries be circumscribed, and let the change
of conditions continue progressing (forming new stations), in such a case the
original inhabitants must cease to be as perfectly adapted to the changed
conditions as they were originally. It has been shown in a former part of this
work, that such changes of external conditions would, from their acting on the
reproductive system, probably cause the organization of those beings which were
most affected to become, as under domestication, plastic. Now, can it be
doubted, from the struggle each individual has to obtain subsistence, that any
minute variation in structure, habits, or instincts, adapting that individual
better to the new conditions, would tell upon its vigour and health? In the
struggle it would have a better chance of surviving; and those of its offspring
which inherited the variation, be it ever so slight, would also have a better
chance. Yearly more are bred than can survive; the smallest grain in the
balance, in the long run, must tell on which death shall fall, and which shall
survive. Let this work of selection on the one hand, and death on the other,
go on for a thousand generations, who will pretend to affirm that it would
produce no effect, when we remember what, in a few year, Bakewell effected in
cattle, and Western in sheep, by this identical principle of selection?
To give an imaginary example from changes in progress on an island:--let
the organization of a canine animal which preyed chiefly on rabbits, but
sometimes on hares, become slightly plastic; let these same changes cause the
number of rabbits very slowly to decrease, and the number of hares to increase;
the effect of this would be that the fox or dog would be driven to try to catch
more hares: his organization, however, being slightly plastic, those
individuals with the lightest forms, longest limbs, and best eyesight, let the
difference be ever so small, would be slightly favoured, and would tend to live
longer, and to survive during that time of the year when food was scarcest;
they would also rear more young, which would tend to inherit these slight
peculiarities. The less fleet ones would be rigidly destroyed. I can see no
more reason to doubt that these causes in a thousand generations would produce
a marked effect, and adapt the form of the fox or dog to the catching of hares
instead of rabbits, than that greyhounds can be improved by selection and
careful breeding. So would it be with plants under similar circumstances. If
the number of individuals of a species with plumed seeds could be increased by
greater powers of dissemination within its own area (that is, if the check to
increase fell chiefly on the seeds), those seeds which were provided with ever
so little more down, would in the long run be most disseminated; hence a
greater number of seeds thus formed would germinate, and would tend to produce
plants inheriting the slightly better-adapted down [*].
Besides this natural means of selection, by which those individuals are
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 change 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 RUSSEL 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 few 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
exterminations; 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 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.
The variety would now have replaced the species, of which it would be a
more perfectly developed and more highly organized form. It would be in all
respects better adapted to secure its safety, and to prolong its individual
existence and that of the race. Such a variety could not return to the
original form; for that form is an inferior one, and could never compete with
it for existence. Granted, therefore, a "tendency" to reproduce the original
type of the species, still the variety must ever remain preponderant in
numbers, and under adverse physical conditions again alone survive. But this
new, improved, and populous race might itself, in course of time, give rise to
new varieties, exhibiting several diverging modifications of form, any of
which, tending to increase the facilities for preserving existence, must, by
the same general law, in their turn become predominant. Here, then, we have
progression and continued divergence deduced from the general laws which
regulate the existence of animals in a state of nature, and from the undisputed
fact that varieties do frequently occur. It is not, however, contended that
this result would be invariable; a change of physical conditions in the
district might at times materially modify it, rendering the race which had been
the most capable of supporting existence under the former conditions now the
least so, and even causing the extinction of the newer and, for a time,
superior race, while the old or parent species and its first inferior varieties
continued to flourish. Variations in unimportant parts might also occur,
having no perceptible effect on the life-preserving powers; and the varieties
so furnished might run a course parallel with the parent species, either giving
rise to further variations or returning to the former type. All we argue for
is, that certain varieties have a tendency to maintain their existence longer
than the original species, and this tendency must make itself felt; for though
the doctrine of chances or averages can never be trusted to on a limited scale,
yet, if applied to high numbers, the results come nearer to what theory
demands, and, as we approach to an infinity of examples, become strictly
accurate. Now the scale on which nature works is so vast--the numbers of
individuals and periods of time with which she deals approach so near to
infinity, that any cause, however slight, and however liable to be veiled and
counteracted by accidental circumstances, must in the end produce its full
legitimate results.
Let us now turn to domesticated animals, and inquire how varieties
produced among them are affected by the principles here enunciated. The
essential difference in the condition of wild and domestic animals is
this,--that among the former, their wellbeing and very existence depend upon
the full exercise and healthy condition of all their senses and physical
powers, whereas, among the latter, these are only partially exercised, and in
some cases are absolutely unused. A wild animal has to search, and often to
labour, for every mouthful of food--to exercise sight, hearing, and smell in
seeking it, and in avoiding dangers, in procuring shelter from the inclemency
of the seasons, and in providing for the subsistence and safety of its
offspring. There is no muscle of its body that is not called into daily and
hourly activity; there is no sense or faculty that is not strengthened by
continual exercise. The domestic animal, on the other hand, has food provided
for it, is sheltered, and often confined, to guard it against the vicissitudes
of the seasons, is carefully secured from the attacks of its natural enemies,
and seldom even rears its young without human assistance. Half of its senses
and faculties are quite useless; and the other half are but occasionally called
into feeble exercise, while even its muscular system is only irregularly called
into action.
Now when a variety of such an animal occurs, having increased power or
capacity in any organ or sense, such increase is totally useless, is never
called into action, and may even exist without the animal ever becoming aware
of it. In the wild animal, on the contrary, all its faculties and powers being
brought into full action for the necessities of existence, any increase becomes
immediately available, is strengthened by exercise, and must even slightly
modify the food, the habits, and the whole economy of the race. It creates as
it were a new animal, one of superior powers, and which will necessarily
increase in numbers and outlive those inferior to it.
Again, in the domesticated animal all variations have an equal chance of
continuance; and those which would decidedly render a wild animal unable to
compete with its fellows and continue its existence are no disadvantage
whatever in a state of domesticity. Our quickly fattening pigs, short-legged
sheep, pouter pigeons, and poodle dogs could never have come into existence in
a state of nature, because the very first step towards such inferior forms
would have led to the rapid extinction of the race; still less could they now
exist in competition with their wild allies. The great speed but slight
endurance of the race horse, the unwieldy strength of the ploughman's team,
would both be useless in a state of nature. If turned wild on the pampas, such
animals would probably soon become extinct, or under favourable circumstances
might each lose those extreme qualities which would never be called into
action, and in a few generations would revert to a common type, which must be
that in which the various powers and faculties are so proportioned to each
other as to be best adapted to procure food and secure safety,--that in which
by the full exercise of every part of his organization the animal can alone
continue to live. Domestic varieties, when turned wild, must return to
something near the type of the original wild stock, or become altogether
extinct.
We see, then, that no inferences as to varieties in a state of nature can
be deduced from the observation of those occurring among domestic animals. The
two are so much opposed to each other in every circumstance of their existence,
that what applies to the one is almost sure not to apply to the other.
Domestic animals are abnormal, irregular, artificial; they are subject to
varieties which never occur and never can occur in a state of nature: their
very existence depends altogether on human care; so far are many of them
removed from that just proportion of faculties, that true balance of
organization, by means of which alone an animal left to its own resources can
preserve its existence and continue its race.
The hypothesis of Lamarck--that progressive changes in species have been
produced by the attempts of animals to increase the development of their own
organs, and thus modify their structure and habits--has been repeatedly and
easily refuted by all writers on the subject of varieties and species, and it
seems to have been considered that when this was done the whole question has
been finally settled; but the view here developed renders such an hypothesis
quite unnecessary, by showing that similar results must be produced by the
action of principles constantly at work in nature. The powerful retractile
talons of the falcon- and the cat-tribes have not been produced or increased by
the volition of those animals; but among the different varieties which occurred
in the earlier and less highly organized forms of these groups, those always
survived longest which had the greatest facilities for seizing their prey.
Neither did the giraffe acquire its long neck by desiring to reach the foliage
of the more lofty shrubs, and constantly stretching its neck for the purpose,
but because any varieties which occurred among its antitypes with a longer neck
than usual at once secured a fresh range of pasture over the same ground as
their shorter-necked companions, and on the first scarcity of food were thereby
enabled to outlive them. Even the peculiar colours of many animals, especially
insects, so closely resembling the soil or the leaves or the trunks on which
they habitually reside, are explained on the same principle; for though in the
course of ages varieties of many tints may have occurred, yet those races
having colours best adapted to concealment from their enemies would inevitably
survive the longest. We have also here an acting cause to account for that
balance so often observed in nature,--a deficiency in one set of organs always
being compensated by an increased development of some others--powerful wings
accompanying weak feet, or great velocity making up for the absence of
defensive weapons; for it has been shown that all varieties in which an
unbalanced deficiency occurred could not long continue their existence. The
action of this principle is exactly like that of the centrifugal governor of
the steam engine, which checks and corrects any irregularities almost before
they become evident; and in like manner no unbalanced deficiency in the animal
kingdom can ever reach any conspicuous magnitude, because it would make itself
felt at the very first step, by rendering existence difficult and extinction
almost sure soon to follow. An origin such as is here advocated will also
agree with the peculiar character of the modifications of form and structure
which obtain in organized beings--the many lines of divergence from a central
type, the increasing efficiency and power of a particular organ through a
succession of allied species, and the remarkable persistence of unimportant
parts such as colour, texture of plumage and hair, form of horns or crests,
through a series of species differing considerably in more essential
characters. It also furnishes us with a reason for that "more specialized
structure" which Professor Owen states to be a characteristic of recent
compared with extinct forms, and which would evidently be the result of the
progressive modification of any organ applied to a special purpose in the
animal economy.
We believe we have now shown that there is a tendency in nature to the
continued progression of certain classes of varieties further and further from
the original type--a progression to which there appears no reason to assign any
definite limits--and that the same principle which produces this result in a
state of nature will also explain why domestic varieties have a tendency to
revert to the original type. This progression, by minute steps, in various
directions, but always checked and balanced by the necessary conditions,
subject to which alone existence can be preserved, may, it is believed, be
followed out so as to agree with all the phenomena presented by organized
beings, their extinction and succession in past ages, and all the extraordinary
modifications of form, instinct, and habits which they exhibit.
Ternate, February, 1858.
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[*] This MS. work was never intended for publication, and therefore was
not written with care.--C.D. 1858.
[*] I can see no more difficulty in this, than in the planter improving
his varieties of the cotton plant.--C.D. 1858.
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