ARE SPECIES STABLE? 4! 



species, very much alike in other respects, may have 

 their sexual organs very dissimilar, and sterility 

 would be the result, while two other species, or va, 

 rieties, while very unlike in most features, might 

 have their sexual organs so similar as to produce 

 perfect fertility. If this is true, we reach a new dis- 

 tinction of species and varieties. The differences 

 between varieties may affect any part of the body 

 except the sexual organs. Differences between 

 species include, as a rule, also the sexual organs. 



It is now possible to see some reason why our 

 domestic races continue fertile. These races have 

 been produced by selection on the part of the 

 breeders, who do not pay any attention to the sex- 

 ual organs in their selection. The general differ- 

 ences may therefore become very great without 

 producing sterility. Moreover, there is very good 

 reason for believing that domestication has the 

 effect of making many animals more readily suscep- 

 tible to cross-breeding. Our various breeds of dogs, 

 for instance, are almost certainly descended from 

 several wild stocks which must once have been dis- 

 tinct species. But to-day, all dogs, with the excep- 

 tion of certain South American indigenous species, 

 breed freely with each other. Now either these 

 wild stocks originally could interbreed, or, what is 

 more probable, they have acquired the power by 

 domestication. If it is thus true that domestication 

 sometimes produces greater fertility in this way, 

 another suggestion is found in the fact for the fer- 

 tility of domestic races. 



In spite of all these facts and suggestions, this 



