DIVERGENCE RESULTING FROM ISOLATION. 253 



selection, and thai all diversity of selection is due to exposure to 

 different environments. The divergences in the cases above referred 

 to, it is said, are probably due to differences in the environment that 

 are not easily recognized. This was the explanation suggested by 

 Darwin when the facts were reported to him in 1N72. The division of 

 a species into isolated portions did not seem to him to furnish any 

 factor that could produce divergence unless it was aided by exposure 

 to different external conditions. The same view is expressed in his 

 "Origin of Species," sixth edition, page 319. 



My reply is twofold. (1) The theory that all divergences in Sand 

 wich Island land mollusks are due to differences in the environment 

 requires us to believe that there are occult influences increasing in 

 difference with each additional mile of separation, and that these 

 influences control the natural selection of the mollusks, but have no 

 influence on any other species occupying the same areas. A theory 

 that involves so heavy an assumption can not be received when a 

 simpler theorv is open to us. (2) I believe I can entirely remove this 

 objection, urged against my conclusion on these purely theoretical 

 grounds, bv showing that there are certain causes of divergence, not 

 depending on exposure to different environments, that are necessarily 

 introduced by the division of a species into isolated groups ; and that, 

 under the influence of these causes, diversity of habits ma}- arise pro 

 during diversity of selection, even while the fragments are exposed to 

 the same environment. 



I have elsewhere called attention to the fact that the independent 

 breeding of separated groups, as far as we can judge, always tends to 

 produce divergence; and I have shown that, when a species is indis- 

 criminately broken into independent fragments, the tendency to diver- 

 gence will, on the average, vary in direct proportion to the instability 

 of the species and in inverse proportion to the size of the fragments; 

 for on these factors depends the probable degree of departure of the 

 average character of the fragment from the average character of the 

 species previous to its being broken into fragments, and, therefore, 

 the degree of segregation. 



I wash now to show that the maintenance of certain classes of char- 

 acters alwavs belonging to an unbroken species is due to a form of 

 selection that can continue only so long, and so far, as free crossing 

 continues. Reflexive selection is a formative principle, depending on 

 the relations in which the members of an intergenerating group of 

 organisms stand to each other, while they continue to intergenerate ; 

 but when two portions of an original species have become so divergent 

 as to compete with each other in the same area without crossing, they 



