On the Tendency of Varieties to Depart Indefinitely From the Original Type (1858)
by Alfred Russel Wallace
ne
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 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.
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 well-being 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.
[ Alfred Russel Wallace, "On the Tendency of Varieties to Depart
Indefinitely from the Original Type," part of the joint Darwin-Wallace paper
entitled "On the Tendency of Species to form Varieties, and on the Perpetuation
of Varieties and Species by Natural Means of Selection," Jour. of the
Proc. of the Linnean Society (Zoology), 3 (July 1858): 53-62. ]
Home Page |
Further Reading |
Site Map |
Send Feedback
