INTENSIVE SEaEEGATION. 357 . 



segregation ; while those that are most manifestly held apart 

 hy sexual instincts and im-pregnationalincompatabilities do 

 not folloiv this law. 



Bidivrella is represented by two groups of species, cue of ovate 

 form, the other elongated and with the outlines of the spire less 

 rounded. The widest divergence between tliese groups is pre- 

 sented by species occupying the same districts and valleys, but 

 the widest divergences iu the species of either of these groups 

 are found in valleys widely separated. In the latter case, the 

 degree of geographical separation is probably an approximate 

 measure of the time and degree of segregation, and therefore the 

 measure of the degree of divergence ; while, in the former case, 

 the segregation is probably as complete between forms occupying 

 the same valley as between those of widely separated valleys. 

 There is reason to believe that in the eastern part of the island 

 these two groups are not held apart by sexual segregation or 

 segregate fecundity and vigour, for there is complete iutergradiug, 

 and the divergence between the groups in any one valley is much 

 less than is found in the north-west portion of the island, where 

 sexual incompatibility seems to hold them apart. 



Achatinella hacca and A. abhreviata completely intergrade with 

 each other, but they are a.ssociated with a number of other species 

 o? Achatinella with which they do not intergrade, prevented it 

 seems to me by mutual antipathy and sterility. We have, there- 

 fore, in the eastern valleys two groups of Achatinella completely 

 segregated from each other, though occupying the same districts 

 and in some measure the same stations ; while in the other valleys 

 the two groups coalesce, the different species occupying any one 

 valley being only partially segregated by divergent habits of feeding 



The different subgenera, which are undoubtedly segregated by 

 divergent sexual instincts, as well as by physiological incompati- 

 bilities, are equally divergent, whether we compare forms from 

 the same, or from distant valleys. 



8. The distribution of the varieties, species, and genera of 

 Achatinella on this island is just such as would be produced 

 by divergent evolution, which depends on segregation as a 

 necessary condition even when the environments are different, 

 and which always follows long-conti7iued segregation even 

 when the environment surrounding the different sections is 

 the same. 



