575.8 181 
Ill 
Reproductive Divergence: An Additional Factor 
in Evolution 
~\OME ten years ago the late G. J. Romanes propounded his theory 
of Physiological Selection,’ 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? 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 Romanes. 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. 
