'J'ELEGONY AND REVERSION. 127 



for zebra hybrids differing from their striped parents by 

 discontinuous variation or not. As, however, reversion 

 towards a remote ancestor in the case of the Equidas at 

 least has not been proved — is probably not even sus- 

 ceptible of proof, — it is most desirable that we should 

 endeavour to learn as much as possible from zebra hybrids 

 for and against the reversion doctrine. 



Theoretically, as already mentioned, the result of crossing 

 distinct varieties or species should be the production of an 

 intermediate form. This ideal result is seldom attained. 

 It seems, however, that, as Liebscher has shown, the ideal 

 result is sometimes practically reached, e. g. when the two 

 species of barley, Hordeiim steudellii ? and 11. trifurca- 

 .tum ^ , are crossed, a hybrid is obtained whicli is as nearljr 

 as possible intermediate between the two parent forms.* 



But usually, as Darwin long ago pointed out, the cross- 

 bred offspring, instead of combining in equal proportions 

 the characters of the sire and dam, may radically differ 

 from the iramediate ancestors and often closely resemble 

 known or supposed remote ancestors. By inbreeding, the 

 carriers of heredity or, let us say, the germ units respon- 

 sible for maintaining the distinctive characters of any 

 given race are fixed and it may be strengthened. Hence, 

 when two inbred and closely related forms are mated, the 

 offspring are likely to closely resemble their immediate 

 parents. In cross-breeding exactly the opposite holds 

 true ; it is quite uncertain what the offspring will be 

 like, unless one of the parents is highly prepotent. For 

 example, if two closely related thoroughbreds are mated 

 the offspring will almost certainly resemble the parents or 

 the grandparents — the resemblance being greatest to the 

 most inbred parent. But if a thoroughbred is mated with 

 a cross-bred mare — with, say, a country-bred Indian mare — 

 it is impossible to say what the progeny will be like ; it may 

 be large but quite devoid of quality, or small and fine, 

 have plenty of quality but no substance. By inbreeding 

 and by a form of selective breeding (whicli though quite a 

 * Liebscber's work is referred to in Weismann's 'Germ-plasm,' p. 301. 



