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frequently on the various kinds of dun. They may sometimes plainly be seen on foals and subsequently disappear.
"The dun color and the stripes are strongly transmitted when a horse thus characterized is crossed with any other, but I was not able to prove that striped duns are generally produced from the crossing of two distinct breeds, neither of which are duns, although this does sometimes occur.
"The legs of the ass are often striped, and this may be considered as a reversion to the wild parent form, the Annus tceniopus of Abyssinia, which is thus striped. In the domestic animal the stripes on the shoulder are occasionally double or forked at the extremity, as in certain zebrine species. There is reason to believe that the foal is frequently more plainly striped on the legs than the adult animal. As with the horse, I have not acquired any distinct evidence that the crossing of differently colored varieties of the ass brings out the stripes.
"But now let us turn to the result of crossing the horse and ass. Although mules are not nearly so numerous in England as asses, I have seen a much greater number with striped legs, and with the stripes far more conspicuous than in either parent form. Such mules are generally light-colored, and might be called fallow-duns. The shoulder stripe in one instance was deeply forked at the extremity, and in another instance was double, though united in the middle. Mr. Martin gives a figure of a Spanish mule with strong zebra-like marks on its legs, and remarks that mules are particularly liable to be thus striped on the legs. In South America, according to Eoulin, such stripes are more frequent and conspicuous in the mule than in the ass. In the United States, Mr. Gosse, speaking of these animals, says that in a great number, perhaps in nine out of every ten, the legs are banded with transverse dark stripes."
Mules with striped legs can be seen in great numbers every day in the streets of Baltimore, and the peculiarity is not in the least uncommon.
Darwin gives a number of cases in which the same reversion has been produced by the crossing of other horselike forms, and we must regard the tendency to revert to a striped form, when crossed as characteristic of the horse family.
Darwin says that when he crossed different varieties of fowls he often got birds with faint traces of the peculiar red plumage of the wild Gallus hankiva, and that this plumage was almost perfectly reproduced in one magnificent bird, the offspring of a black Spanish cock and a white silk hen, although either of these pure breeds may be reared by tens of thousands without the appearance of a single red feather.
Even long-lost instincts may be made to reappear by crossing. The original wild ancestor of our domestic fowls must, like all wild incubating birds, have had the incubating instinct. Now when two non-sitting breeds of fowls are crossed, the mongrels frequently recover their incubating habit and sit with remarkable steadiness.
It is said that hybrids between perfectly tame domestic animals are often as wild as their wild ancestors. This has been noticed in cattle, pigs, fowls, ducks, and it is probable that the same thing frequently shows itself when widely separated human races are crossed, as such good authorities as Livingston and Humboldt have remarked upon the savage character of half caste human beings.
Another interesting resemblance between reversion and ordinary variation is the fact that the descendants of hybrids are more apt to revert than the hybrids themselves. Darwin says (Variation, p. 65) that this is a general rule.
Now, whether reversion be due to the sudden excitement of a tendency which has long been transmitted in a dormant state by the ova, or whether it is due to the appearance of a new variation which resembles an old one, we can readily understand how, according to our theory of heredity, crossing should call this power into action. During the evolution of the species each hereditary peculiarity has been established in the egg by gemmules, and anything which prevents the egg from following its normal course and develo2:>ing the recently acquired characteristics of the species, would allow older characteristics to appear in their place.
We know that animals which are very widely separated are infertile, and we can understand that even when the difference between two species is not great enough to prevent them from crossing, those cells of their bodies which have varied most may be so different from each other that gemmules from the one cannot fertilize the egg-particles which are to produce the other, or when they do fertilize them they may give rise to a variation which is so different from the normal cell that it cannot live. The cells which precede these in the order of growth being less different in the two parents, would be much more favorably situated, and would thus give to the embryo a characteristic of longer standing than the peculiarities of either parent. On the other hand, if reversion is simply variation, we can see that crossing might excite reversion just as it excites variability.
Summary of Cliapter.
The study of hybrids gives us a means of comparing, within certain narrow limits, the parts which the two sexual elements play in heredity. The influence of each sex can, in a certain sense, be studied by itself when a given species is used in the one case as the father of a hybrid, and in another case as the mother. The value of crossing as an experiment in heredity is greatly limited, however, by the fact that, although we can study the influence of one sexual element unobscured by the other element from the same species, it is obscured and complicated by the influence of this element from an allied species, and in all organisms which can breed together the reproductive elements must be essentially alike.
Hybrids do, however, present a number of peculiarities which agree perfectly with what we should expect according to our hypothesis, and certain of these are inexplicable without it.
Hybrids and mongrels are highly variable, as we should expect to be the case, according to Darwin's pangenesis hypothesis. This hypothesis fails to account for the fact that hybrids from forms which have long been domesticated are more variable than those from wild species or varieties, or for the very remarkable fact that the children of hybrids are much more variable than the hybrids themselves.
Our theory not only explains the variability of hybrids, but it also accounts for the two latter peculiarities, for crossing will not give rise to a marked or conspicuous variation unless the hybrid inherits numbers of gemmules, and as domesticated animals and plants live under unnatural conditions they are more favorably placed than wild forms for the production of gcmmules. The body of a hybrid is in itself a new thing, and therefore in a certain sense unnatural, and a male hybrid is, accordingly, more fitted for the production of gemmules than a male of a pure or unmixed race.
When a male hybrid is crossed with the female of either pure species or with a third species, the children are much more variable than those born from a hybrid mother by a male of a pure species. It would be difficult to devise an experiment better fitted than this to show that variation is caused by the influence of the male, and that the action of unnatural or changed conditions upon the male parent results in the variability of the child.
The remarkable history of reciprocal hybrids is directly opposed to Darwin's view that the functions of the two reproductive elements are essentially similar, for in some cases it is impossible to breed from a female of one species by the male of a second species, while the male of the first species readily fertilizes the ovum of the second and gives rise to fertile offspring. Even when both crosses are fertile the one is often much more so than the other.
The hybrids of one cross often differ remarkably from those of the other cross in general structure, and in many cases they show, in addition to the common characteristics of both parents, a tendency, more or less perfectly pronounced, to develop the recently acquired characteristics of,that species which is used as the father.
This law is often obscured by the appearance of reversions, which are peculiarly apt to occur in hybrids, and by the presence, in certain cases, of a tendency for each parent to transmit its peculiarities to the hybrid, without fusion with those of the other parent. But when we