575.8 317 



II 



Reproductive Divergence : A Factor in Evolution ? 



IN the September number of Natural Science (p. 181) Mr H. M. 

 Vernon propounds a new theory, called Reproductive Divergence, 

 which, he says, is essentially different from Romanes' Physiological 

 Selection. Inasmuch as both Reproductive Divergence and Physio- 

 logical Selection are a process, not a cause, are based on the occur- 

 rence of the same kind of variation among the individuals of a 

 species, and have the same end in view, the differences between the 

 two principles do not appear to me to be of great importance ; the 

 premise of Reproductive Divergence is, however, more general, and 

 the way in which the principle is demonstrated is certainly inde- 

 pendent. Reproductive Divergence (like Physiological Selection) is 

 brought forward to show that under certain propositions given 

 differences between the individuals of a species inhabiting the same 

 locality and presumed to stand under the same external influences 

 will develop into specific differences solely by means of Reproductive 

 Divergence. What we have to understand by specific- differences 

 is quite clear in this case : it is that kind of difference which we 

 find to exist between two morphologically very closely allied forms 

 which, though existing together in the same locality, are entirely 

 independent of one another, the two forms (1) breeding true, the 

 one never producing an individual that belongs to the other, and (2) 

 never fusing into one form, in spite of their not being mutually 

 absolutely sterile. In another place x I have referred at some 

 length to Physiological Selection, and endeavoured to show that this 

 principle does not hold good in so far as the outcome of Physio- 

 logical selection, as propounded by Romanes, is, at best, dimorphism, 

 not specific distinctness ; and I came to the conclusion that forms 

 deviating from each other in the way just mentioned cannot be 

 evolved without the aid of some kind of local separation. 2 It does 

 not seem to me that Mr Vernon's arguments in support of Repro- 

 ductive Divergence as a factor in the evolution of specific distinct- 

 ness are any more valid than those which were adduced by Romanes 

 in favour of Physiological Selection. The occurrence of such a 



1 " Novitates Zoologicae," 1896, p. 426 if. 



2 Local races are now generally termed subspecies ; their high significance, especially 

 in questions of general Biology, will doubtless be recognised in time also by those 

 systematists who still persist in ignoring subspecies. 



