222 APPENDIX II — INTENSIVE SEGREGATION. 



Bulimella is represented by two groups of species, one of ovate form, 

 the other elongated and with the outlines of the spire less rounded. 

 The widest divergence between these groups is presented by species 

 occupying the same districts and valleys, but the widest divergences in 

 the species of either of these groups are found in valleys widely sepa- 

 rated. In the latter case, the degree of geographical separation is prob- 

 ably an approximate measure of the time and degree of isolation, and, 

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

 case, the isolation 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 fully held apart by sexual isolation or segregate fecundity 

 and vigor, for there is complete intergrading, and the divergence 

 between the groups in any one valley is much less than is found in the 

 northwest portion of the island, where sexual incompatibility seems 

 to be complete. 



Achatinella bacca and A. abbreviata completely intergrade with each 

 other, but they are associated with a number of other species of Acha- 

 tinella with which they do not intergrade, prevented, it seems to me, by 

 mutual incompatibility. We have, therefore, 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 isolated 

 by divergent habits of feeding. 



The different genera and subgenera, which are undoubtedly segre- 

 gated by divergent sexual instincts, as well as by physiological incom- 

 patibilities, are equally divergent, whether we compare forms from 

 the same or from distant valleys. 



(8) The distribution of the varieties, species, and genera of Achati- 

 nella on this island is just such as would be produced by divergent evolu- 

 tion which depends on isolation as a necessary condition, even when the 

 environments are different, and which always follows long-continued 

 isolation, even when the environment surrounding the different sections 

 is the same. 



It may be safely said of the multitude of varieties which inhabit the 

 island of Oahu that every one is more or less segregated from all other 

 varieties ; and I believe this will be found true concerning varieties 

 in every part of the world. This fundamental fact would probably 

 never have been denied, except for the delusive idea that the advan- 

 tage of divergence would lead to the accumulation of divergence 

 even if segregation were entirely wanting. What could be a greater 



