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 it will prove 
impossible for them to impregnate each other owing to the lack of 
correlation in their forms. I 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 will 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 any 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 codperating with structural isolation. 
Let 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 definiteness 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) Ifa colony 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, 
