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, for example, 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 (Aleuritcs 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. 



