|
Wallace, Alfred Russel. 1858 Linnean Society of London, Proceedings On the Tendency of Varieties to Depart Indefinitely from the Original Type Instability of Varieties supposed to prove 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 of 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 Struggle for Existence. 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 Law of Population of Species. 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 he 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 difference
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 much 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 Useful Variations will tend to Increase; useless or hurtful Variations to Diminish. 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 Superior Varieties will ultimately Extirpate the original Species. 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 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. *That is, they will vary, and the variations which tend to
adapt them to the wild state, and therefore approximate them
to wild animals, will be preserved. Lamarck's Hypothesis very different from that now advanced. We see, then, that no inferences as to varieties in a state
of nature can be deduced from the observation of those occurring
among Conclusion. We believe we have now shown that there is a tendency in nature
to the continued progression of certain classes of varieties
further Written at Ternate, February, 1858 |
|