575.8 404 
[December 
Vv 
Reproductive Divergence: A Rejoinder 
N the last month’s number of Natural Science (p. 317) Dr Karl 
Jordan criticises a theory which I had briefly suggested in a 
previous number of the journal (p. 181), and which I had entitled 
“ Reproductive Divergence: an Additional Factor in Evolution.” I 
had there maintained that my theory differed essentially from 
Romanes’ theory of Physiological Selection, for I endeavoured to 
show that if the less similar individuals in any species were at the 
same time less fertile znter se than the more similar, it would neces- 
sarily follow that in the course of succeeding generations these 
members would diverge more and more from each other, till eventu- 
ally two or more new and mutually sterile species would be formed. 
I still hold, in spite of Dr Jordan’s view to the contrary, that this 
theory is essentially different from Romanes’, which maintains that 
if a portion of the members of a species happen to be sterile with all 
the other members, they will, in virtue of this physiological barrier, 
be enabled to vary independently of the parent stock, and so give 
rise to a new species. 
I stated that my theory was made up of two parts, one of which 
was capable of mathematical demonstration, whilst the other could 
only be verified by experiment. Dr Jordan takes exception to the 
former, but accepts the latter, he holding that the “correlation between 
morphological characters and fertility of the specimens of a species 
cannot be denied.” He also adduces an additional instance in support 
ef the existence of this correlation. 
The mathematical demonstration of the validity of the theory 
which I gave was, it would seem, rather too brief for its purpose. 
In excuse I must plead that my paper was intentionally of only a 
short and preliminary nature, as I thought a more extended discus- 
sion had better be deferred till I had more experimental evidence at 
my command. As, however, Dr Jordan appears to have entirely 
misunderstood my reasoning, he holding indeed that in the particular 
example I adduced to prove a divergence of character there would 
on the contrary be a convergence, I must now endeavour to explain 
the mathematical basis of the theory more fully. Thus of its validity 
I am convinced there can be no question. Its adequate demon- 
stration depends only on skill in manipulation of figures, though 
this I am afraid I do not possess. 
