292 THE THEORIES OF EVOLUTION 



two different varieties falls on the same pistil; this 

 pistil exercises, so to speak, its power of selection; a 

 certain exclusive crossfertilisation occurs which finally 

 results in a new type. Many other cases could be in- 

 stanced of sexual antipathy between varieties of one 

 species and of slight modifications in the structure or 

 activity of organs, etc. 



Vernon has formulated a theory which he calls the 

 theory of reproductive divergence in the following 

 words: "Supposing that among the members of any 

 species those individuals more alike in respect to any 

 different characteristic, such as colour, form or size, 

 are slightly more fertile inter se than less similar indi- 

 viduals, it necessarily follows that in the course of suc- 

 ceeding generations the members of this species will 

 diverge more and more in respect to the characteristic 

 in question, whereby the original species may be ulti- 

 mately split up into two or more fresh species." As 

 a concrete example, Vernon supposes that in the 

 Lepidopterous Ithania Urolina, sl certain insect found 

 in the Amazon Valley, small individuals were slightly 

 more fertile when mated with other small individuals 

 than when mated with large individuals, while these 

 were also more fertile inter se. "Then it would fol- 

 low that fewer individuals of intermediate size would 

 be produced, and in the course of time the species 

 would be split up into a small and large variety." 7 



t H. M. Vernon. Reproductive Divergence: An Additional Factor 

 in Evolution. (Nat. Sc, 1897, p. 181.) 



