STRUCTURAL ISOLATION AND STRUCTURAL SELECTION. 69 



left side of the body. The first is called a dextral variety, and bears 

 a dextral shell. Each individual has both male and female organs, 

 and any two dextral individuals easily unite and impregnate each 

 other, as do also any two sinistral individuals. But if a dextral 

 individual and a sinistral are brought together, I think il will prove 

 impossible for them to impregnate each other owing to the lack of 

 correlation in their forms. 1 earnestly hope that in the Hawaiian 

 Islands, where there are not a few species represented by both dextral 

 and sinistral varieties, careful investigation of this point will be made. 



I anticipate that crossing between dextral and sinistral forms will 

 be found to be impossible. If this is so, it is probable that if, in a 

 group of one form occupying one tree, there arise- in the same genera- 

 tion two or more individuals of the reverse form from the original 

 stock, they will mate; and there wall be formed, without intergrading 

 steps, a completely segregated group, determined in the first place by 

 structural isolation. It should, however, be noted that when the rep- 

 resentatives of anv species found on any one tree are all either dextral 

 or sinistral, any single individual of the reverse form that may appear 

 in any generation will be prevented from leaving offspring, and the 

 result will be structural selection, a form of reflexive selection deter- 

 mined by and cooperating with structural isolation. 



L,et us now consider whether environal selection is one of the causes 

 that produces both dextral and sinistral varieties occupying the same 

 valley and often sharing the same groves or the same individual 

 trees, and for the sake of defmiteness let us suppose that the original 

 form entering the valley was dextral. We then ask: 



(1) Did the two or more sinistral individuals originating the new- 

 type gain any advantage from their sinistral form when they first 

 appeared ? 



(2) Does the colony as it now exists derive any advantage from 

 their form that they would not equally enjoy if they had all remained 

 in the original dextral form? 



(3) If a eolonv of sinistral individuals occupying a given candlenut 

 tree and a colony of dextral individuals occupying another tree of the 

 same species are made to exchange trees, will each group find them- 

 selves unfitted for the new position? 



My observations on dextral and sinistral varieties of Hawaiian snails 

 lead me to believe that in every case all three of these questions 

 should be answered in the negative; and that, therefore, the forma- 

 tion of this distinction can not be attributed to natural selection, nor, 

 indeed, primarily to any form of selection. After the new form has 

 been produced by variation and preserved by structural isolation, 



