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Show 10 VARIATION CHAP. I. Such buds can be propagated by grafting, &c., and sometimes by seed. These "sports" are ex~ren:ely rare under nature, but far from rare under cultivation; and in this case we see that the treatment of the parent has affected a bud or offset, and not the ovules or pollen. But it is the opinion of most physiologists that the::e is ~o essential difference between a bud and an ovule In their earliest st.ages of formation ; so that, in fact, " sports~' support my view, that variability may be largely attributed to the ovules or pollen, or to both, having been affected by the treatment of the parent prior to t~e ~ct ~f conception. These cases anyhow show that var1at1on IS not necessarily connected, as some authors have supposed, with the act of generation. Seedlings from the same fruit, and the young of the same litter, sometimes differ considerably from each other, though both the young and the parents, as MUller has remarked, have apparently been exposed to exactly the same conditions of life; and this shows how unimportant the direct effects of the conditions of life are in comparison with the laws of reproduction, and of growth, and of inheritance; for had the action of the conditions been direct, if any of the young had varied, all would probably have varied in the same manner. To judge how much, in the case of any variation, we should attribute to the direct action of heat, moisture, light, food, &c., is most difficult: my impression is, that with animals such agencies have produced very little direct effect, though apparently more in the case of plants. Under this point of view, Mr. Buckman's recent experiments on plants seem extremely valuable. When all or nearly all the individuals exposed to certain conditions are affected in the same way, the change at :first appears to be directly due to such conditions ; but in some cases it can be shown that quite opposite conditions produce CHAP. r. UNDER DOMESTICATION. 11 similar changes of structure. Nevertheless some slight a~ount of change may, I think, be attributed to the ~Irect actio~ of the conditions of life-as, in some cases, ~~creased. size from amount of food, colour from parti~ ular kinds of food and from light, and perhaps the thickness of fur from climate. Habit also has a decided influence, as in the period of. flowering with plants when transported from one chmate to another. In animals it has a more marked effect ; for instance, I find in the domestic duck that the bones .of the wi~g weigh less and the bones of the leg more, In proportion to the whole skeleton, than do the same bones in the wild-duck; and I presume that this change may be safely attributed to the domestic duck flying much less, and walking more, than its wild parent. The great and inherited development of the udders in co_ws and goats in countries where they are habitually ~rulked, in comparison with the state of these organs In other countries, is another instance of the effect of use. Not a single domestic animal can be named ":hich has not in some country drooping ears; and the VIew suggested by some authors, that the drooping is due to the disuse of the muscles of the ear, from the animals not being much alarmed by danger, seems probable. Th~re are many laws regulating variation, some few of which can be dimly seen, and will be hereafter briefly mentioned. ~ will here only allude to what may be called correlation of growth. Any change in the embryo or larva will almost certainly entail changes in the mature animal. In monstrosities, the correlations be~ ween quite distinct parts are very curious ; and many Instances are given in Isidore Geoffroy St. Hilaire's great work on this subject. Breeders believe that long limbs are almost always accompanied by an elongated head. Some instances of correlation are quite whimsical: thus |
