Page:Encyclopædia Britannica, Ninth Edition, v. 24.djvu/94

Rh 78 VARIATION descended like varieties from other species. Nevertheless, such a conclusion, even if well founded, would be unsatis factory until it could be shown how the innumerable species inhabiting this world have been modified so as to acquire that perfection of structure and co-adaptation which justly excites our admiration.&quot; Again, &quot;It is therefore of the highest importance to gain a clear insight into the means of modification and co-adaptation. At the commence ment of my observations it seemed to me probable that a careful study of domesticated animals and cultivated plants would offer the best chance of making out this obscure problem. Nor have I been disappointed : in this and all other perplexing cases I have invariably found that our knowledge, imperfect though it be, of variation under domestication affords the best and safest clue.&quot; Hence the first chapter is devoted to an account of those ex tensive studies of variation under domestication which later formed the subject of a separate work. Varia- A comparison between individuals of the same variety of culti- tion vated plants or animals shows a greater degree of variation than under between individuals of any one species or variety in nature. This domesti- higher variability of domestic productions is to be ascribed to the cation, less uniform conditions of their upbringing, perhaps in part also to excess of food. Exposure to new conditions must be continued for generations to set up any great variation ; but this, once set up, con tinues indefinitely. Changed conditions may directly influence the whole organization or certain parts alone, or act indirectly throiigh the reproductive system. With respect to the direct action, the nature of the organization seems more important than that of the conditions. The effect on offspring may be definite : e.g., size may depend upon the amount of food, colour upon quality of food, thick ness of skin and hair upon climate, &c. But indefinite variability is a much commoner result of changed conditions, and has probably played a much more important part in the formation of our domestic races. The reproductive system is peculiarly sensitive to very slight external changes. Many plants and animals will not reproduce in domestication, even though individually vigorous ; others, though weak and sickly, breed freely. Hence we need not be surprised at the reproductive system acting irregularly and producing -variations. But that variation is not exclusively associated with sexual repro duction is demonstrated by the case of plants &quot;sporting&quot; through bud - variations. Such cases, moreover, prove the nature of the organism to be of more importance than the conditions. Changed habits and use and disuse produce an inherited effect ; witness the lighter wing- and heavier leg-bones of the domestic duck, the enlarged udders of milch-cows, or the drooping (unpricked) ears of domestic mammals. Variations are often definitely correlated : short-beaked pigeons have small feet ; hairless dogs have imperfect teeth ; and blue-eyed white tom-cats are deaf. Hence selection of any one character will probably modify others indirectly. Although the laws governing inheritance are mostly unknown, probably most, if not all, characters tend to be inherited. The popular statement that domestic varieties, when they run wild, revert to the primitive stock is unsupported by satisfactory evidence ; and, although rever sions occasionally occur in domestication, there is no sign of any general tendency to the loss of acquired characters. Except in being less uniform than natural species, in often differing more widely in some single part, and in being fertile when crossed, there are no well-marked distinctions between our domestic races and the so-called true species of a genus. The many breeds of dogs and cattle may arise from more than one species ; but probably those of horses and fowls, and clearly those of rabbits, ducks, and pigeons, are each descended from a single species. In this respect the breeds of pigeons are of peculiar importance, since not only carrier and tumbler, runt and barb, pouter and fantail, but at least a score of varieties might be chosen which differ so thoroughly, internally as well as externally, that an ornithologist, treating them as wild birds, would be compelled to grant them specific, and even distinct generic rank. Yet, since all these have indisputably arisen from the wild rock -dove (Columba livid), it is clear that naturalists who admit a unity to such domestic races, which professed breeders have often laughed to scorn, should in turn be cautious before de riding the unity of wild ones. How then have domestic races been produced ? By external con ditions or habits alone ? One of their most remarkable features is in exhibiting adaptation, not to their own good, but to man s use or fancy. We know that all the breeds were not produced as per fect as we now see them, and the key is man s power of accumu lative selection : nature gives successive variations ; man adds them up, making for himself useful breeds. Skilful breeders speak of the organization as plastic and under control, and have effected ex tensive modifications within our own generation. Unconscious selection, which results from every one trying to possess and breed the best individuals, is even more important. This accumulated change explains why we so often cannot recognize the wild parent stocks of our cultivated plants, while its absence in countries in habited by uncivilized man explains why these never yield plants worth immediate culture. Man s power of selection is facilitated by keeping large numbers, in which variations are more likely to occur. Facility in preventing crosses is also of importance, as of pigeons as contrasted with cats ; some species are, however, less variable than others, e.g., the goose. Individual differences arise even in the offspring of the same Varia- parents and tend to be inherited ; hence they afford material for tion natural selection to act on and accumulate, precisely as they would under for human selection. (Polymorphic genera e.g., Rosa, Riibus, nature. Hieracium, &c. may perhaps owe their protean character to their variations being of no service or disservice, and consequently not being acted on by natural selection.) In determining whether a form should be ranked as species or variety, the opinion of natural ists of sound judgment and wide experience is the only guide, yet this lacks unanimity (see SPECIES) : for example, of the polymorphic genera of the British flora alone Bentham reckons 112 species but Babington 251. Wallace has shown that no certain criterion can possibly be given by which to define his convenient working cate gories of Malayan Lepidoptera, &c., viz., variable forms, local forms, sub-species, and representative species. As De Candolle concluded from his monograph on oaks (in which he shows at least two-thirds of his 300 species to be provisional), &quot;so long as a genus is imper fectly known and its species founded upon a few specimens &quot; they seem clearly limited ; but, &quot;jnst as we come to know them better, intermediate forms flow in and doubts as to specific limits augment.&quot; The terms variety and species are thus arbitrarily applied to inde finable groups of more or less closely similar individuals. Common species that range wide and are much diffused are those which vary most. The species of the larger genera in each country vary more frequently than the species of the smaller genera. The species of large genera present strong analogies with varieties, which we can only understand if they originated as such. The term &quot; struggle for existence &quot;is used in a wide sense, in- Struggle eluding dependence of one being upon another, and embracing for exist- (which is more important), not only the life of the individual, but ence. success in leaving progeny. From the high (geometrical) rate of increase of all organic beings (slow breeders only requiring a few more years to people a whole district) struggle inevitably follows, either one individual with another of the same species, or with the individuals of a distinct species, or with the physical conditions of life. It is the doctrine of Mai thus applied with manifold force to the entire animal and vegetable kingdoms, for in this case there can be no artificial increase of food and no prudential restraints from marriage. The checks to increase are most obscure. Eggs or young animals generally suffer most, and plants, mostly as seed lings, both from germinating on ground already occupied and from animals. The amount of food, of course, gives the extreme limit of numbers, very frequently, however, the attacks of enemies, as of game by vermin. Climate plays an important part, and periodi cal seasons of extreme cold have destroyed as many as four-fifths of the birds of an observed area. Epidemics, too, may occur. In many a species a large stock of individuals is often essential to its preservation. Complex and unexpected checks and relations exist between organic beings which have to struggle together ; witness the profound alteration of the flora and fauna of a heath when planted with Scotch fir, these again being wholly dependent upon the exclusion of cattle. But in several parts of the world insects determine the existence of cattle. Again, red clover depends for fertilization upon the humble-bees, these upon immunity from the attacks of field-mice, and thus indirectly upon the numbers of cats ; hence no bees, no clover, and the more cats, the more clover ! The struggle will almost invariably be most severe between the indi viduals of the same species, for they frequent the same districts, require the same food, and are exposed to the same dangers. In the case of varieties of the same species the struggle will generally be almost equally severe, and we sometimes see the contest soon de cided (as in the case of varieties of wheat or sweet pea, of the mountain-sheep or medicinal leech). Similarly, the struggle be tween species of the same genus will generally be more severe than between the species of distinct genera, e.g., the replacement of the black rat by the brown or of the large cockroach by the small. The structure of every being is related to that of all the others with which it competes, from which it escapes, or on which it preys ; witness alike the teeth and talons of the tiger, or the legs and claws of the parasite clinging to his hair. The albumen of a seedling favours its struggle with plants already growing around it. How will this struggle for existence act in regard to variation ? Natural Can the principle of selection, so potent in the hands of man, apply selection. under nature ? Most efficiently ; for, when we bear in mind the constant occurrence of variation, with the strength of the heredi tary tendency, also how infinitely close and complex are the mutual relations of organic beings to each other and to their physical con-