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Show 408 CONCLUDING REMARKS. CIIAP. XXVIII. they often ignore the whole subject of geographical distribution as completely as if its laws were the res~t ?f chance .. Although from the reasons just assigned 1t ~s often drfficu~t to judge accurately of the amount of change whiCh our dom~sticated productions have undergone, yet this can be ascertamed in the cases in which we know that all the breeds are descended from a single species, as with the pigeon, duck, rabbit, a~cl ~lmost certainly with the fowl; and by the aiel of analogy tlns IS to a certain extent possible in the case of animals descended from several wild stocks. It is impossible to read the details given in the earlier chapters, and in many published works, or to visit our various exhibitions, without being deeply impressed with the extreme variability of our domesticated animals and cultivated plants. I have in many instances purposely given details on new and strange peculiarities which have arisen. No part of the organisation escapes the tendency to vary. The variations generally affect parts of small vital or physiological importance, but so it is with the differences which exist between closely allied species. In these unimportant characters there is often a greater difference between the breeds of the same species than between the natural species of the same genus, as Isidore Geoffroy has shown to be the case with size, and as is often the case with the colour, texture, form, &c., of the hair, feathers, horns, and other dermal appendages. 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 vertebrre. 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 vertebrre and the forms of the cervical vertebrre 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 oosophagus, have all varied. In certain quadrupeds the length of the intestines has been much increased or CHAP. XXVIII. CONCLUDING REMARKS. 409 diminished. With plants we see wonderful differences in the stones of various fruits. In the Cucurbitacere 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 clown 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 |
