248 DARWINISM TO-DAY. 



opposite property, fertility, is of vital importance to every 

 species, and gives the offspring of the individuals which 

 possess it, in consequence of their superior numbers, a 

 greater chance of survival in the battle of life. It is, there- 

 fore, indirectly under the control of natural selection, which 

 acts both for the self-preservation of fertile and the self- 

 destruction of unfertile stocks except always, as correlated 

 above, when they become useful and therefore subject to be 

 increased by natural selection." Jordan maintains that the 

 outcome of physiological selection can be at best only 

 dimorphism, not specific distinctness. 



Vernon 21 has formulated a theory which he calls that of 

 "reproductive divergence," in the following words: "Sup- 

 Vemon's posing that among the members of any species 

 ductive^er- " tnose individuals more alike in respect to any 

 gence. 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 succeed- 

 ing generations the members of this species will diverge 

 more and more in respect to the characteristic in question, 

 whereby ultimately the original species may be split up into 

 two or more fresh species." As a concrete example, Vernon 

 supposes that in the Lepidopterous Ithania urolina, a certain 

 insect found in the Amazon Valley, small individuals were 

 slightly more fertile with other small individuals than with 

 large individuals, while these were also more fertile inter se; 

 "then it would follow that fewer individuals of intermediate 

 size would be produced, and in course of time the species 

 would be split up into a small and large variety. These 

 varieties would continue to diverge as long as the process 

 of 'reproductive divergence' was acting, till at length they 

 might become differentiated in the two mutually sterile 

 species. Supposing on the other hand this variation in 

 fertility were correlated with slight differences of colour, 

 then in course of time varieties differing in respect of colour 



