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Show 444 EMBRYOLOGY. CHAP. XIII. acquired a little earlier or later in life. It would not signify, for instance, to a bird which obtained its food best by having a long beak, whether or not it assumed a beak of this particular length, as long as it was fed by its parents. Hence, I conclude, that it is quite possible, that each of t~e many succ~ssiv~ modifications, by which each species has acquired Its present structure, may have supervened at a not very early period of life ; and some direct evidence from our domestic animals supports this view. But in other cases it is quite possible that each successive modification, or roost of them, may have appeared at an extremely early period. I have stated in the first chapter, that there is some evidence to render it probable, that at whatever age any variation first appears in the parent, it tends to reappear at a corresponding age in the offspring. Certain variations can only appear at corresponding ages, for instance, peculiarities in the caterpillar, cocoon, or imago states of the silk-moth; or, again, in the horns of almost full-grown cattle. But further than this, variations which, for all that we can see, might have appeared earlier or later in life, tend to appear at a corresponding age in the offspring and parent. I am far from meaning that this is invariably the case; and I eould give a good many cases of variations (taking the word in the largest sense) whieh have supervened at an ear~ier age in the ehild than in the parent. These two principles, if their truth be admitted, will, I believe, explain all the above specified leading facts in embryology. But first let us look at a few analogous cases in domestic varieties. Some authors who have written on Dogs, maintain that the greyhound and .b~lldog, though appearing so different, are really vanetws most closely allied, and have probably descended from CnAP • .XIII. EMBRYOLOGY. 445 the same wild stock ; hence I was curious to see how far their puppies differ:d fro~ each other : I was told by breeders that they differed JUSt as much as their parents and this, judging by the eye, seemed almost to be th~ case ; but on actually measuring the old dogs and their six-days old puppies, I found that the puppies had not nearly aequired their full amount of proportional difference. So, again, I was told that the foals of eart and race-horses differed as much as the full-grown animals· and this surprised me greatly, as I think it probable that the di:fferenee between these two breeds has been wholly caused by seleetion under domestication; but having had eareful measurements made of the dam and of a three-days old colt of a raee and heavy eart-horse, I find that the colts have by no means acquired their full amount of proportional d.ifferenee. As the evidence appears to me conclusive, that the several domestic. breeds of Pigeon have deseended from one wild species, I compared young pigeons of various breeds, within twelve hours after being hatehed; I earefully measured the proportions (but will not here give details) of the beak, width of mouth, length of nostril a~d of eyelid, size of feet and length of leg, in the wild. stock, in pouters, fantails, runts, barbs, dragons, carriers, and tumblers. Now some of these birds, when mature, differ so extraordinarily in length and form of beak, that .they would, I cannot doubt, be ranked in distincf'genera, had they been natural productions. But when the nestling birds of these several breeds were placed in a row, though most of them could be distinguishe~ from each other, yet their proportional differences In the above specified several points were incomparably less than in the full-grown birds. Some characteristic points of difference-for instance that of the width of mouth-could hardly be detected in the |