STERILITY OF HYBRIDS 123 



condition must necessarily lead to rapid and complete de- 

 generacy. 



Darwin, nevertheless, quite admits that hybridism, as a general 

 rule, results in sterility, and that the further apart two species 

 are, the more difficult their crossing becomes, until at last crossing 

 between species belonging to two distinct famihes is an im- 

 possibility. It is true that a hybrid has once been obtained from 

 a cross between Asterias (a starfish) and Arhacia (a sea-urchin), 

 which belong to widely separated families, but the offspring 

 which resulted did not survive the larval stage. ^ This is the only 

 occasion in which a cross between two distinct families has been 

 obtained. 



Darwin remarks that " with our domesticated animals the 

 various races, when crossed together, are quite fertile ; yet in 

 many cases they are descended from two or more wild species. 

 From this fact we must conclude, either that the aboriginal 

 parent species at first produced perfectly fertile hybrids, or that 

 the hybrids subsequently reared under domestication became 

 quite fertile. This latter alternative seems by far the most 

 probable, and can, indeed, hardly be doubted. It is, for instance, 

 certain that our dogs are descended from several wild stocks ; 

 yet, with perhaps the exception of certain indigenous domestic 

 dogs of South America, aU are quite fertile together." ^ It is 

 certain that domestication tends to counterbalance the difficulty 

 of crossing which different species exhibit in the state of nature ; 

 for instance, two wild species in the state of nature will not cross, 

 or, if they do cross, they will produce either no progeny or sterile 

 progeny ; whereas the lineal descendants of either ancestor, 

 after a prolonged period of domestication, may exhibit a high 

 degree of fertility. 



The degree of fertility, both of first crosses and of hybrids, 

 may vary from zero to perfect fertility. " When pollen from a 



1 Delage, L'Heredite, etc., p. 272. 

 ^ The Origin of Species, p. 374. 



