56 Darwin, and after Darwin. 



down any changes which natural selection, or any 

 other cause of varietal divergence, may attempt to 

 produce ; and therefore, in order to produce — or to 

 increase — such divergence in the absence of any other 

 form of isolation, natural selection must hit upon such 

 changes of structure, form, or colour, as are so cor- 

 related with the reproductive system as to create the 

 physiological isolation that is required. 



To show how the principle of selective fertility 

 may be combined with what apparently is the most 

 improbable form of isolation for this purpose — the 

 geographical — I quote the following suggestion made 

 by Professor Lloyd Morgan in his Animal Life and 

 Intelligence : — 



Suppose two divergent local varieties were to arise in 

 adjacent areas, and were subsequently (by stress of competition 

 or by geographical changes) driven together into a single 

 area. ... If their unions be fertile, the isolation will be an- 

 nulled by intercrossing — the two varieties will form one mean or 

 average variety. But if the unions be infertile, the isolation 

 will be presei'ved, and the two varieties will continue separate. 

 Suppose now, and the supposition is by no means an improb- 

 able 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 whicji had become 

 infertile would remain isolated. Hence, in the long run, iso- 

 lated forms occupying a common area would be infertile, 

 (p. 107.) 



If then cross-sterility may thus arise even in associ- 

 ation with geographical isolation, may it not also 

 arise in its absence ? And may it not thus give rise 

 to the differentiation of varieties on account of this 

 physiological isolation alone? 



