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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 bull-dog, though appearing so different, are really varieties closely allied, and have probably descended from the same wild stock; hence I was curious to see how far their puppies differed from 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 the case; but on actually measuring the old dogs and their six-days old puppies, I found that the puppies had not nearly acquired their full amount of proportional difference. So, again, I was told that the foals of cart and race-horses differed as much as the full-grown animals; and this surprised me greatly, as I think it probable that the difference between these two breeds has been wholly caused by selection under domestication; but having had careful measurements made of the dam and of a three-days old colt of a race and heavy cart-horse, I find that the colts have by no means acquired their full amount of proportional difference.
As the evidence appears to me conclusive, that the several domestic breeds of Pigeon have descended from one wild species, I compared young pigeons of various breeds, within twelve hours after being hatched; I carefully measured the proportions (but will not here give details) of the beak, width of mouth, length of nostril and 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 distinct genera, had they been natural productions. But when
These two principles, namely, that slight variations generally appear at a not very early period of life, and are inherited at a corresponding not early period, explain, as I believe, .. all the above specified leading facts in embryology. But first let us look to a few analogous cases in our domestic varieties. Some authors who have written on Dogs, maintain that the greyhound and bulldog, though appearing so different, are really .. closely allied varieties, descended from the same wild stock; hence I was curious to see how far their puppies differed from each other: I was told by breeder that they differed just as much as their parents, and this, judging by the eye, seemed almost to be the case; but on actually measuring the old dogs and their six-days old puppies, I found that the puppies had not acquired nearly their full amount of proportional difference. So, again, I was told that the foals of cart and race-horses— breeds which have been almost wholly formed by selection under domestication— differed as much as the full-grown animals; ... but having had careful measurements made of the dams and of .. three-days old colts of race-horses and heavy cart-horses, I find that this is by no means the case.
As we have conclusive evidence that the .. breeds of the Pigeon are descended from a single wild species, I compared the young within twelve hours after being hatched; I carefully measured in the wild parent-species, in pouters, fantails, runts, barbs, dragons, carriers, and tumblers, the proportions (but will not here give the details) of the beak, width of mouth, length of nostril and of eyelid, size of feet and length of leg. .. .. .. .. .. .. .. .. .. .. .. .. .. Now some of these birds, when mature, differ in so extraordinary a degree in the length and form of beak, and in other characters, that they would certainly be ranked as distinct genera if found in a state of nature. But when