356 EEV. J. T. &ULICK ON 



chatina, which are for the most part found on the ground under 

 dead and decaying leaves, seem to possess better opportunities 

 for migration than either Acliatinella or Bulimella. Corresponding 

 with these facts we find the species of Acliatinella and Bulimella 

 especially limited in the areas they occupy, while the species of 

 Apex,Amastra, and Leptachatina are less so. For exam pie, the area 

 occupied by Amastra turritella, A. tristis, and A. ventulus includes 

 the areas occupied by many species oi Achatinella and Btdimella ; 

 and Apex loratus and A.pallidus, occupying the mountain-ridges, 

 rant^e from Makiki to Halawa, exceeding the range attained by 

 any arboreal species occupying the valleys of the same region. 



6. When a group of divergent forms that are fertile ivith each 



other are being developed through the influence of local or 

 geographical segregation, other conditions remaining con- 

 stant, the numher of forms that will he produced loithin a 

 given area loill vary inversely as the square of the average 

 radius of distribution for the different forms. 

 As this average radius of distribution may be taken as the 

 measure of the power and opportunities for migration, we may 

 say that other powers and opportunities remaining constant, the 

 number of species developed within a given area will vary inversely 

 as the square of the power and opportunity for migration. 



Though migration is in one sense a cause of isolation, it is 

 evident that the number of isolated groups of individuals does 

 not increase with the increase of migration. Isolation is pro- 

 duced by the great contrast between ordinary and extraordinary 

 combinations of opportunities for migration; and this contr-ast 

 is as great in the case of species that have limited powers and 

 opportunities, as in the case of those that have very great powers 

 and opportunities. The number of isolations thus produced that 

 can exist within the limits of a given area musb vary inversely 

 as the square of the power and opportunity for migration. 



The facts of distribution we have been considering seem to 

 correspond to this law. 



7. Fo7'ms that are most nearly related, and are therefore the 



least subject to sexual and impregnational segregation, are 

 distributed in such a manner that their divergence is directly 

 proportional to their distance from each other, which is 

 also the measure of the time and degree of their geographical 



