DIVERGENCE DEPENDS ON ISOLATION. 223 
mistake for the breeder of animals than to imagine that by selecting 
extreme variations and breeding them together he would in time 
secure well-marked races? It must be equally at variance with fact 
* to suppose that any advantage secured by divergent variations can 
be preserved and accumulated while the different forms are freely 
intergenerating. 
In the family we are considering, the chief forms of isolation are 
probably what I have called local, geographical, industrial, and sexual 
isolation, strengthened in many cases by segregate fecundity and vigor. 
As illustrating local isolation I would mention varieties and species 
of Apex, for the most part occupying mountain ridges which are all 
connected with each other, without the intervention of geographical 
barriers. Geographical isolation is illustrated in the forms of Achati- 
nella and Bulimella, which for the most part occupy the deep valleys, 
the ridges forming barriers that are very rarely surmounted. Indus- 
trial isolation is illustrated by the closely allied varieties of one group 
of species that occupy one valley, but are prevented from crossing by 
different habits of feeding. It is probable that sexual or seasonal iso- 
lation prevents the pairing of Achatinella with Bulimella when both 
occupy the same trees. Moreover, cross sterility would undoubtedly 
prevent the multiplication of the hybrids, if cross-unions ever do occur 
between forms so widely divergent. There can be no doubt that the 
same principles prevent the strongly marked groups of either genus 
from intergenerating; as, forexample, in the case of Achatinella bacca 
and A. abbreviata, which are intergraded with each other, but not 
with the surrounding species of Achatinella. 
Again, divergent forms of environal selection do not necessarily 
depend on exposure to different environments. Industrial isolation 
is produced by different modes of using the environment found in a 
single district ; and the same cause will often produce diversity in the 
forms of environal selection affecting the isolated sections, distrib- 
uted in different districts, but exposed to the same environment. 
Cumulative divergence in the methods of using the environment in 
the different sections of the species depends upon their isolation, 
and, therefore, increasing divergence in the forms of selection affect- 
ing the different branches depends upon their isolation. Geograph- 
ical isolation under the same environment, if it does not of itself 
produce divergent forms of selection, opens the way for change in the 
habits of feeding, with diversity of selection in the different sections of 
the species. Take, for example, the species of Achatinella: In Manoa 
and Makiki they chiefly occupy the kukui (Aleurttes triloba) and other 
trees, while in Kawailoa and that region they neglect the larger trees 
and take to the lobelia and other shrubs and herbaceous plants. 
