575.8 404 [December 



Reproductive Divergence : A Rejoinder 



IN 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 

 " Eeproductive Divergence : an Additional Factor in Evolution." I 

 had there maintained that my theory differed essentially from 

 Eomanes' 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 inter 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 Eomanes', 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 

 of 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 1 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 1 do not possess. 



