358 KKV. J. T. GULICK ON 



Increasing difference in the forms of natural selection does 

 not necessarily depend on exposure to different environments, 

 but does depend on some form of Independent Greneration. 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 advantage of divergence would lead to the accumu- 

 lation of divergence even if segregation were entirely wanting. 

 "What could be a greater 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 advan- 

 tage secured by divergent variations can be preserved and ac- 

 cumulated while the different forms are freely intergeuerating. 



In the family we are considering, the chief forms of segregation 

 are probably what I have called local, geographical, industrial, 

 and sexual segregation, strengthened in many cases by segregate 

 fecundity and vigour. As illustrating local segregation I would 

 mention varieties and species oiApex, for the most part occupying 

 the mountain -ridges which are all connected with each other, 

 without the intervention of geographical barriers. Greographical 

 segregation is illustrated in the forms of Achatinella and Buli- 

 mella, which for the most part occupy the deep valleys, the ridges 

 forming barriers that are very rarely surmounted. Industrial 

 segregation is illustrated by the closely- allied varieties of one 

 group of species that occupy one valley, but are prevented from 

 freely crossing by different habits of feeding. It is probable 

 that sexual or seasonal segregation prevents the pairing ofAcJiati- 

 nella with Bulimella when both occupy the same trees. More- 

 over, 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 inter- 

 generating; as for example, in the case of Achat itiella hacca and 

 A. abbreviafa, which are intergraded with each other, but not with 

 the surrounding species of Acliutinella. 



Again, divergent forms of natural selection do not neccf^sarily 

 depend on exposure to different environments. Industrial Se- 

 gregation is produced by different methods of using the environ- 



