575.8 181 



III 



Reproductive Divergence : An Additional Factor 



in Evolution 



SOME ten years ago the late G. J. Komanes propounded his theory 

 of Physiological Selection, 1 which was founded on the fact that 

 certain individuals of a species, though fertile with some, may be 

 perfectly sterile with other individuals. Supposing such incom- 

 patibility to run through a whole race, then these varieties, 

 separated by a physiological barrier from the rest of the members 

 of the species, would be preserved, and might vary independently, 

 and so become gradually split off from the parent species in respect 

 of other characteristics as well. 



This theory has not been generally received, and Wallace, in 

 particular, has demonstrated 2 very clearly that in the form - pro- 

 pounded by its author the theory cannot stand. Nevertheless, the 

 theory served to draw attention to the importance of variations in 

 the reproductive powers of organisms as a factor in evolution, and 

 to emphasise certain unexplained difficulties in the theory of natural 

 selection, more especially with reference to the sterility of first 

 crosses between species, coupled with the fertility of those between 

 varieties, the swamping effects of intercrossing, and the frequent 

 inutility of specific characters. 



In the present paper I wish to bring forward a theory which is 

 also concerned with variations in the reproductive powers of organ- 

 isms as an important factor in evolution, but which is essentially 

 different from that propounded by Eomanes. This theory may be 

 enunciated as follows. Supposing that among the members of 

 any species, those individuals, more alike, in respect of 

 any characteristic, such as colour, form or size, are slightly 

 more fertile inter se than less similar individuals, it 

 necessarily follows that in the course of succeeding 

 generations the members of this species will diverge more 

 and more in respect of the characteristic in question, 

 whereby ultimately the original species may be split up 

 into two or more fresh species. 



This principle I have ventured to call " Reproductive Diverg- 

 ence." It is best illustrated by a concrete example. Supposing 



1 Journ. Linn. Soc. (Zool.), vol. xix., p. 337, 1886. 



2 "Darwinism," p. 180. 



