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Show HYBRIDISM. [CHAP. VIll. 242 . . . t ongly in one sex than in mithng hkeness runnmg more .s r. ed with another t h e ot he r, b ot h wh e.n one. spemes dIS cr'othss another variety.' and when one vanety IS crosse WI . h . F or I.n s t ance, I thi'nk those authors are nght,t hw ho main-tain that the ass has a prepotent power over e orse, so that both the mule and the hinny more resemble the ass th the horse· but that the prepotency runs more str~~ 1 in the ;nale-ass than in the female, so that t~e mule~~ hich is the offspring of the ma~e-a~s and mare1 Is re like an ass than is the hinny, which IS the offspnng mofo t he female-ass' and sta1 11' on. Much stress has been laid by some autho"fs on the sup-osed fact that mongrel animals alone are born ·closell tke one of their parents ; but it ~an be shown that this does sometimes occur with hybrid~ ; yet I grant much less frequently with hybrids than With mongrels. Loo~irig to the cases which I have collected of cross-bred ammals closelr resembling one parent the resemblan~es see:r;n chiefly confined to characters almost monstrous In theu nature and which have suddenly appeared-such a.s albinis~, melanism, deficiency of tail or horns, or additional fingers and toes ; and d? not relate t? characters which have been slowly acquned by selection. Consequently, sudden reversions to ~he perfect cha:r:acter of either parent would be more hkely to occur w1th mongrels, which are de~cended from. varieties often sudde~ly roduced and semi-monstrous In charac~er, than w1th hvbrids, which are descended from spe~1es slo·wly a~d naturally produced. On the whole I eJ?-tirely agree Wlth Dr. Prosper Lucas, who, after. arranging an enormous body of facts with respect to animals, comes to. the co_n· elusion that the laws of resemblance of the cl!1ld to 1ts parent~ are the same, whether the ~wo parent.s d1ffer. m~c~ or little from each other, namely Ill the u1u~n ?f IndiVIduals of the same variety, or of different varieties, or of distinct species. · .1. . Laying aside the question of fertility and steri Ity, m aU other respects there seems to be a ~eneral and close similarity in the offspring of crossed spemes, and of cros~ef ·varieties. If we look at species as having been speCla · OBAP. VIII.] SUMMARY. 243 ly ·created, and at varieties as having been produced by secondary laws, this similarity would be an astonishing fact. But it harmonises perfectly with the view that there is no essential distinction between species and varieties. Summary of Ohapter.-First crosses between forms sufficiently distinct to be ranked as species, and their hybrids, are very generally, but not universally, sterile. The sterility is of all degrees, and is often so slight that the two most careful experimentalists who have ever lived, have come to diametrically opposite conclusions in ranking forms by this test. The sterility is innately variable in individuals of the same species, and is eminently susceptible of favourable and unfavourable conditions. The degree of sterility does not strictly follow systematic affinity, but is governed by several curious and complex laws. It is generally different, and sometimes widely different, in reciprocal crosses between the same two species. It is not always equal in degree in a first cross and in the hybrid produced from this cross. In the same manner as in grafting trees, the capacity of one species or variety to take on another, is incidental on generally unknown differences in their vegetative systems, so in crossing, the greater or less facility of one species to unite with another, is incidental on unknown differences in their reproductive system. There is no more reason to think that species have been specially endowed with various degrees of sterility to prevent them crossing and blending in nature, than to think that trees have been specially endowed with various and somewhat ~nalogous degrees of difficulty in being grafted together In order to prevent t1?-e1n becoming inarched in our forests. The sterility of first crosses between pure species, which have their reproductive systems perfect, seems to depend on several circumstances ; in some cases largely on the early death of the embryo. The sterility of hybrids, '~hi<;h have thei; reproductive s:ystmns imperfect, and whiCh have had th~s system and thmr whole organisa .. |
