136 SECRETS OF EARTH AND SEA 



species, and (2) how that is to say, by what structure or 

 by what subtle chemical differences or other features in 

 their make-up and habit are they prevented from so 

 hybridizing ? Then we come on further to the question, 

 Why should a hybrid, once produced, fail to bear healthy 

 eggs or sperms according to its sex, although it grows up 

 to full size and is to all appearances mature? And why 

 should hybrids between parents of origin locally remote 

 from one another not show this failure, but behave like 

 ordinary healthy organisms? 



In the full solution of these inquiries we should get 

 very near to some of the most important secrets of the 

 living body which have still to he searched out. But a 

 reply to these questions which is probably in large 

 measure true, and serves to help us in the further 

 collection and examination of facts, is as follows : First, 

 the production and maintenance of " species " of plants 

 and of animals by survival of favourable variations in the 

 struggle for existence (Darwin and Wallace's theory of 

 the origin of species) requires the maintenance of the purity 

 of the favourable stock which survives in the struggle. If 

 it were continually liable to hybridization by other species 

 it would never establish its own distinctive features. It 

 would deteriorate by departing from those characteristics 

 which have been " naturally selected " and have rendered 

 it a successful "species." Thus the breeder, when he 

 has selected a stock for propagation which approaches 

 the standard at which he is aiming, keeps it apart, and 

 does not allow it to be " crossed " by other stock. One of 

 the qualities " naturally selected " in " the wild " is the 

 power of resistance to fertilization by neighbouring 

 species. 



This power of resistance or immunity to fertilization 

 by other species may be attained by several different 

 methods. Amongst these are (i) a difference ^ in the 



