It has often been asserted that important parts never vary under domestication, but this is a complete error. Look at the skull of the pig in any one of the highly improved breeds, with the occipital condyles and other parts greatly modified; or look at that of the niata ox. Or again, in the several breeds of the rabbit, observe the elongated skull, with the differently shaped occipital foramen, atlas, and other cervical vertebræ. The whole shape of the brain, together with the skull, has been modified in Polish fowls; in other breeds of the fowl the number of the vertebræ and the forms of the cervical vertebræ have been changed. In certain pigeons the shape of the lower jaw, the relative length of the tongue, the size of the nostrils and eyelids, the number and shape of the ribs, the form and size of the œsophagus, have all varied. In certain quadrupeds the length of the intestines has been much increased or
diminished. With plants we see wonderful differences in the stones of various fruits. In the Cucurbitaceæ several highly important characters have varied, such as the sessile position of the stigmas on the ovarium, the position of the carpels within the ovarium, and its projection out of the receptacle. But it would be useless to run through the many facts given in the earlier chapters.
It is notorious how greatly the mental disposition, tastes, habits, consensual movements, loquacity or silence, and the tone of voice have varied and been inherited with our domesticated animals. The dog offers the most striking instance of changed mental attributes, and these differences cannot be accounted for by descent from distinct wild types. New mental characters have certainly often been acquired, and natural ones lost, under domestication.
New characters may appear and disappear at any stage of growth, and be inherited at a corresponding period. We see this in the difference between the eggs of various breeds of the fowl, and in the down on chickens; and still more plainly in the differences between the caterpillars and cocoons of various breeds of the silk-moth. These facts, simple as they appear, throw light on the characters which distinguish the larval and adult states of natural species, and on the whole great subject of embryology. New characters are liable to become attached exclusively to that sex in which they first appeared, or they may be developed in a much higher degree in the one than the other sex; or again, after having become attached to one sex, they may be partially transferred to the opposite sex. These facts, and more especially the circumstance that new characters seem to be particularly liable, from some unknown cause, to become attached to the male sex, have an important bearing on the acquirement by animals in a state of nature of secondary sexual characters.
It has sometimes been said that our domestic productions do not differ in constitutional peculiarities, but this cannot be maintained. In our improved cattle, pigs, &c., the period of maturity, including that of the second dentition, has been much hastened. The period of gestation varies much, but has been modified in a fixed manner in only one or two cases. In
our poultry and pigeons the acquirement of down and of the first plumage by the young, and of the secondary sexual characters by the males, differ. The number of moults through which the larvæ of silk-moths pass, varies. The tendency to fatten, to yield much milk, to produce many young or eggs at a birth or during life, differs in different breeds. We find different degrees of adaptation to climate, and different tendencies to certain diseases, to the attacks of parasites, and to the action of certain vegetable poisons. With plants, adaptation to certain soils, as with some kinds of plums, the power of resisting frost, the period of flowering and fruiting, the duration of life, the period of shedding the leaves and of retaining them throughout the winter, the proportion and nature of certain chemical compounds in the tissues or seeds, all vary.
There is, however, one important constitutional difference between domestic races and species; I refer to the sterility which almost invariably follows, in a greater or less degree, when species are crossed, and to the perfect fertility of the most distinct domestic races, with the exception of a very few plants, when similarly crossed. It certainly appears a remarkable fact that many closely allied species which in appearance differ extremely little should yield when united only a few, more or less sterile offspring, or none at all; whilst domestic races which differ conspicuously from each other, are when united remarkably fertile, and yield perfectly fertile offspring. But this fact is not in reality so inexplicable as it at first appears. In the first place, it was clearly shown in the nineteenth chapter that the sterility of crossed species does not closely depend on differences in their external structure or general constitution, but results exclusively from differences in the reproductive system, analogous with those which cause the lessened fertility of the illegitimate unions and illegitimate offspring of dimorphic and trimorphic plants. In the second place, the Pallasian doctrine, that species after having been long domesticated lose their natural tendency to sterility when crossed, has been shown to be highly probable; we can scarcely avoid this conclusion when we reflect on the parentage and present fertility of the several breeds of the dog, of Indian and European cattle, sheep, and pigs. Hence it would be unreasonable to expect that races formed under domestication
should acquire sterility when crossed, whilst at the same time we admit that domestication eliminates the normal sterility of crossed species. Why with closely allied species their reproductive systems should almost invariably have been modified in so peculiar a manner as to be mutually incapable of acting on each other—though in unequal degrees in the two sexes, as shown by the difference in fertility between reciprocal crosses in the same species—we do not know, but may with much probability infer the cause to be as follows. Most natural species have been habituated to nearly uniform conditions of life for an incomparably longer period of time than have domestic races; and we positively know that changed conditions exert an especial and powerful influence on the reproductive system. Hence this difference in habituation may well account for the different action of the reproductive organs when domestic races and when species are crossed. It is a nearly analogous fact, that most domestic races may be suddenly transported from one climate to another, or be placed under widely different conditions, and yet retain their fertility unimpaired; whilst a multitude of species subjected to lesser changes are rendered incapable of breeding.
With the exception of fertility, domestic varieties resemble species when crossed in transmitting their characters in the same unequal manner to their offspring, in being subject to the prepotency of one form over the other, and in their liability to reversion. By repeated crosses a variety or a species may be made completely to absorb another. Varieties, as we shall see when we treat of their antiquity, sometimes inherit their new characters almost, or even quite, as firmly as species. With both, the conditions leading to variability and the laws governing its nature appear to be the same. Domestic varieties can be classed in groups under groups, like species under genera, and these under families and orders; and the classification may be either artificial,—that is, founded on any arbitrary character,—or natural. With varieties a natural classification is certainly founded, and with species is apparently founded, on community of descent, together with the amount of modification which the forms have undergone. The characters by which domestic varieties differ from each other are more
variable than those distinguishing species, though hardly more so than with certain protean species; but this greater degree of variability is not surprising, as varieties have generally been exposed within recent times to fluctuating conditions of life, are much more liable to have been crossed, and are still in many cases undergoing, or have recently undergone, modification by man's methodical or unconscious selection.