575.8 317 
II 
Reproductive Divergence: A Factor in Evolution ? 
1 ie the September number of Natwral 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! 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.? 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 ff. 
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. 
