HYBRIDS AND MONGRELS 149 



Following the modern usage, we shall apply the 

 term ' hybrid ' to all individuals arising from a cross 

 be*tween parents which belong to distinct groups, no 

 matter whether these groups are separated as distinct 

 genera or species, or whether they are regarded as 

 representing only different races or varieties. This 

 wide interpretation of the term hybrid has only re- 

 cently been reintroduced. The use to which it has 

 returned is, indeed, the original one ; but many inter- 

 mediate writers, including Darwin, confined the em- 

 ployment of this expression to cases of crossing between 

 species, and applied the word ' mongrel ' to the off- 

 spring of crosses between races or varieties of the same 

 species. Darwin, however, did not regard species as 

 differing in kind from varieties, and he even particu- 

 larly emphasized the smallness of the distinction which 

 can be drawn between the behaviour and properties 

 of hybrids and mongrels respectively. Indeed, he 

 came to the highly important conclusion that the laws 

 of resemblance between parents and their children are 

 the same, whatever may be the amount of difference 

 between the parents in question whether, that is to 

 say, they are distinguished only by individual differ- 

 ences, or whether they belong to separate varieties or 

 even species. We have already seen that the more 

 recent facts of biometry point strongly towards the 

 conclusion that individual and race differences are 

 inherited at approximately the same rate. It seems, 

 however, to be at present somewhat doubtful whether 

 all sorts of specific differences follow the same law of 

 propagation on cross-breeding. 



