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Show 264 HYBRIDISM:. CHAP. VIII. male element may reach the female element, but be incapable of causing an embryo to be developed, as seems to have been the case with some of Thuret's experiments on Fuci. No explanation. can be given of these facts, any more than why certain trees cannot be grafted on others. Lastly, an embryo may be developed, and then perish at an e~rly period. This latter alternative has not been suffiCiently attended to; but I believe, from observations communicated to me by Mr. Hewitt, who has had great experience in hybridising gallinaceous birds, that the earl~ ~ea~h of the embryo is a very frequent cause of sterihty In first crosses. I was at first very unwilling to believe in this view ; as hybrids, when once born, are generally healthy and long-lived, as we see in the case of the common mule. Hybrids, however, are differently circumstanced before and after birth : when born and living in a country where their two parents can live, they are generally placed under suitable conditions of life. But a hybrid partakes of only half of the nature and constitution of its mother, and therefore before birth, as long as it is nourished within its mother's womb or within the egg or seed produced by the mother, it may be exposed to conditions in some degree unsuitable, and consequently be liable to perish at an early period; more especially as all very young beings see~ eminently sensitive to injurious or unnatural condi-tions of life. In regard to the sterility of hybrids, in which th.e sexual elements are imperfectly developed, the case IS very different. I have more than once alluded t? a large body of facts, which I have collected, showm.g that when animals and plants are removed from the~r natural conditions, they are extremely liable to have the~r reproductive systems seriously affected. This, in fact, IS CHAP, VITI. CAUSES OF STERILITY. 265 the great bar to the domestication of animals Bet the ste n·1 I· ty t h us superinduced and that of h ·b 'd twhe en . f . . . Y n s, ere ~re. many points o srmilanty. In both cases the sterilit Is Independent of general health and is often y 'db . ' aooo~ pame y exc~s~ of size or great luxuriance. In both cases, the sterility. occurs in various degrees ; in both, the male element Is the most liable to be affected . b t sometimes the female more than the male. In bot~ the ~endency goes to a certain extent with systemati~ affinit~, for whole groups of animals and plants are rendered Impotent by the same unnatural conditions. and whole groups of species tend to produce sterile hybrids. ~n the ?ther hand, one species in a group will sometim?~ resist great c~anges of conditions with unimpaired fertihty ; and certain species in a group will produce unhu suh ally fertile h. ybrids. No one can tell, ti'll h e t ri·e s, w et er any particular animal will breed under confine-ment or any plant seed freely under culture· nor can h~ tell, till he tries, whether any two species ~f a genus Will p~oduc~ more or less sterile hybrids. Lastly, when organic bmngs are placed during several generations under co~ditions not natural to them, they are extreme!! hable to .vary, which is due, as I believe, to their reproductive systems having been specially affected, tho~g~ in .a lesser degree than when sterility ensues. . So It Is With hybrids, for hybrids in successive ~enerati~ns are eminently liable to vary, as every experimentalist has observed. Thus we see that when organic beings are placed under new and unnatural conditions, and when hybrids are produced ?Y the unnatural crossing of two species, the reproductiv~ system, independently of the general state of health, Is affected by sterility in a very similar :anne~. In the one case, the conditions of life have een disturbed, though often in so slight a degree as to N |