Variation and Natural Selection. 107 



sterility of their male offspring, a secondary effect of their 

 segregation? or is their segregation the direct effect of 

 their differential fertility? The former is the general 

 opinion ; the latter is held by Mr. Eomanes. He contends 

 that sterility is the primary distinction of species, other 

 specific characters being secondary, and regards it as a pure 

 assumption to say that the secondary differences between 

 species have been historically prior to the primary differ- 

 ence. I do not propose to discuss this question. While it 

 seems to me in the highest degree improbable that 

 differential fertility, apart from other co-operating factors, 

 has been or could be a practical mode of segregation, it 

 has probably been a not unimportant factor in association 

 with other modes of segregation or isolation. Suppose, for 

 example, two divergent local varieties were to arise in 

 adjacent areas, and were subsequently (by stress of com- 

 petition or by geographical changes) driven together into 

 a single area : we are justified in believing, from the 

 analogy of the Falkland Island cattle, the Forest of Dean 

 deer, and other similar observed habits, that preferential 

 breeding, kind with kind, would tend to keep them apart. 

 But, setting this on one side, let us say they interbreed. 

 If, then, their unions are fertile, the isolation will be 

 annulled by intercrossing — the two varieties will form one 

 mean or average variety. But if the unions be infertile, 

 the isolation will be preserved, and the two varieties will 

 continue separate. Suppose now, and the supposition is 

 by no means an improbable one, that this has taken place 

 again and again in the evolution of species : then it is 

 clear that those varietal forms which had continued to be 

 fertile together would be swamped by intercrossing ; while 

 those varietal forms which had become infertile would 

 remain isolated. Hence, in the long run, isolated forms 

 occupying a common area would be infertile. Or suppose, 

 once more, that, instead of the unions between the two 

 varietal forms being infertile, they are fertile, but give rise 

 to sterile (mule) or degenerate offspring, as is said to be 

 the case in the unions of Japanese and Ainos : then it is 



