Part 2 (1/2)

II.

ON THE TENDENCY OF VARIETIES TO DEPART INDEFINITELY FROM THE ORIGINAL TYPE.[D]

+--------------------------------------------------------------+ | [D] Written at Ternate, February, 1858; and published in the | | Journal of the Proceedings of the Linnaean Society for | | August, 1858. | +--------------------------------------------------------------+

_Instability of Varieties supposed to prove the permanent distinctness of Species._

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 const.i.tute 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 a.s.suming 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 a.n.a.logy 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 a.s.sumption, that _varieties_ occurring in a state of nature are in all respects a.n.a.logous 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 a.s.sumption 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 circ.u.mstances 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._

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; and the wild a.s.ses 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![E] 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 or 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.

+--------------------------------------------------------------+ | [E] This is under estimated. The number would really amount | | to more than two thousand millions! | +--------------------------------------------------------------+

Perhaps the most remarkable instance of an immense bird population is that of the pa.s.senger 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 pa.s.s 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 circ.u.mstances 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 subst.i.tutes 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 reasons why woodp.e.c.k.e.rs 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 gra.s.ses being preserved during the winter, and our farm-yards and stubble-fields furnis.h.i.+ng 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-sh.o.r.es 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 succ.u.mb.

_The Abundance or Rarity of a Species dependent upon its more or less perfect Adaptation to the Conditions of Existence._

It seems evident 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 circ.u.mstances 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 establis.h.i.+ng 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.

_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 could 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 pa.s.senger 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 cla.s.ses--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.