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Show 232 HYBRIDISM. [CHAP. VIIL mental difference between the mere adhesion of graft~d stocks, and the union of the male and _female elen1ents I~ the act of reproduction, yet that t)lere IS a rude d~gree ?~ parallelis1n in the results of graft1ng and of cros~1ng distinct species. And as we must .l?ok ~t the .curious and complex laws governing the. fa~1lity with which tree~ ~a~ be grafted on each other as InCidental or un~{nown diffmences in their vegetative systems, so I beheve that the still ~ore coinplex laws governing ~he facility ?f fir~t crosses are incidental on unknown differences, chiefly 1n their ;eproductive sys~ems. These ~ifferences, in both cases follow to a certain extent, as Inight have been expect~ d, systemat~c ~ffh:;ity, by which ever;r kin~ of r~smnblance and dissimilanty between orgamc beings IS attempted to be expressed. The fact by n~ means sem~s to me to indicate that the greater or lesser drfficulty of either grafting or crossing together :arious species has . been a special endowment ; although In the case of crossmg,, ~he difficulty is as impo~tant for the endur~nce. a~d st~bihty of specific forms, as In tbe case of grafting It IS unimpor-tant for their welfare. • Oauses of the Sterility of forst Crosses and of I-Iyorids.W e may now look a little closer at the probable causes of the sterility of first crosses and of hybrids. These two cases are fundamentally different, for, as just remarked, in the union of two pure species the male and female sexual elements are perfect, whereas in hybrids they are~~perfect. Even in first crosses, the greater or lesser difficulty in effecting an union apparentl:y depends on sev~ral distinct causes. There must sometimes be a physical impossibility in the male elmnent reaching the ovule, as would be the case with a plant having a pistil too long for the pollen tubes to reach the ovarium. It has also been observed that when pollen of one species is placed on the stigma of a distantly allied species, though. the P?llen-t~bes protrude, they do not penetrate the stigmatic surface. Again, the male elemen~ 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 CHAP. VIII.] CAUSES OF STERILITY. 233 experiments on Fuci. No explanation can be given of these facts, any more than why certain trees cannot be grafted on o~hers. Lastly, an .embryo may be developed, and then pensh at an early penod. 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 gallinacedus birds, that the early death of the embryo is a very frequent ca:u~e of steril~ty in. first. cro~ses. I was at first very unWilhng to beheve In this VIew; as hybrids, when once born, are generally healthy and long-lived, as we see in t~e case of ~he common mule. Hybrids, however, are differently cucumstanced before and after brrth : when ~orn and living in a country where their two parents can hve, they are generally placed under suitable conditions of life. ~ut !1 hybr~d partakes of only half of the nature and constitution of Its mother, and therefore before birth as. l~ng as it is nourished within its mother's womb o; w~thin the egg ~r.seed.produced by the mother, it may be exposed to con~Itlons In so~e degree m1suitable, and conseque_ ntly be hable to pensh at an -early period; more e~pema~lr as. all very young beings seem eminently sensitive to InJunous or unnatural conditions of life. In regard to the sterility of hybrids in which the sexual. elements are imperfectly develop~d, the case is very different. I. have more than once alluded to a large bo~y of facts, which I have collected, showing that where a~I_IDals and plants are removed from their natural con~ Itions, they ar~ extremely liable to have their reproductive systems seriou.sly .affected. This, in fact, is the great ~~r to the domestiCation of animals. Between the sterIhty thus superinduced and that of hybrids there are ~any points of similarity. In both cases the' sterility is I~dependent o~ general health, and is often accompanied by ~~cess of size or great luxuriance. In both cases the ster1hty occurs in various degrees ; in both the ::nale element is the most liable to be effected,; but' sometimes the female m~re than th~ male. In both, the tendency goes to a cer.tam extent w1th systematic affinity, for whole groups of animals and plants are rendered impotent by the |