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. Inthe 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 rsland ts qust such as would be produced by divergent evolu- 
tion which depends on rsolation 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 
as 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 
