THE GENESIS OF SPECIES. 177 



The seventh objection recalls the still unexplained 

 physiological difference between ' species ' and ' races,' 

 unions between the former being sterile, and between 

 the latter fertile. In this branch of the subject there 

 is much scope still for inquiry. Some of the difficulty 

 may be due to a trick played us by language. True 

 species have been defined to be those that are not fertile 

 together ; and from the definition it follows that races 

 which are fertile together are not true species. But the 

 question is obscured by the use of the two different 

 words ( races' and ( species,' the real issue being, whether 

 races that are and races that are not fertile together can 

 originate in the same way. The subject in its other 

 bearings has been largely discussed by Mr. Darwin 

 in his work on ' Animals and Plants under Domesti- 

 cation.' 



It remains only to say a few words on the argument 

 from the calculation of chances which is supposed to 

 reduce the survival by natural selection of any particular 

 useful variation almost or altogether to an arithmetical 

 impossibility. * The advantage/ we are told, ( whatever 

 it may be, is utterly outbalanced by numerical inferiority. 

 A million creatures are born : ten thousand survive to 

 produce offspring. One of the million has twice as good 

 a chance as any other of surviving : but the chances are 

 fifty to one against the gifted individual's being one of 

 the hundred survivors. No doubt the chances are twice 

 as great against any one other individual, but this does 

 not prevent their being enormously in favour of some 



