294 THE EVOLUTION THEORY 



human generations. It will also be admitted that our terrestrial 

 snails in Central Europe are much less isolated in their different 

 areas, that, for instance, they could get from one side of a mountain 

 to the other by active migration ; but we must nevertheless repeat the 

 question : how does it happen that in Oahu every forest, every mountain- 

 crest, and so on, has its own variety or species, while our snails are 

 distributed over wide stretches of country, frequently without even 

 developing sharply defined local varieties'? The large vineyard or 

 edible snail {Helix j^oniatia) occurs from England to Turkey, that 

 is, over a distance of about 3,000 kilometres, and within this region 

 it is found in many places which might quite as well be considered 

 isolated as adjacent forest valleys in Oahu. It occurs also on the 

 islands of the Channel and of the Irish Sea, and lives there without 

 intermingling with the members of the species on the mainland. 

 But even on the Continent itself it would be possiljle to name 

 hundreds of places in which they are just as well protected from 

 intermingling with those of other areas as they are in Oahu. There 

 too the snails must somehotu have reached their present habitat 

 some time or other, perhaps rather in an indirect way, by means 

 of other animals ; but this is true also of the snails of a continent, 

 as we shall show more precisely later on. In the meantime let us 

 assume that this is so, and that the vineyard snail (Helix pomatia), or 

 some other widely distributed snail, is relatively isolated. Why then 

 have not hundreds of luell-raarked varieties evolved — a special one for 

 each of the isolated areas ? 



Obviously there must have been something in operation in the 

 Sandwich Islands which is absent from the continental habitats of 

 Helix pomatia, for this species shows fluctuations only in size, but is 

 otherwise the same everywhere, and the few local varieties of it which 

 occur are unimportant. I am inclined to believe that this 'some- 

 thing ' depends on two factors, and especially on the fact that the 

 immigrant snail enters upon a period of variability. This will be 

 brought about in the first place by the fact that the climate and other 

 changes in the conditions of life will call forth a gradually cumulative 

 disturbance in the equilibrium of the determinant system, and thus 

 a variability in various directions and in various combinations of 

 characters. To this must be added the operation of natural selection, 

 which attempts to adapt the immigrant to manj^ new spheres of life, 

 and thus increases in diverse ways the variational tendencies afforded 

 by germinal selection. These two co-operating factors bring the 

 species into a state of' flux or lability, just as a species becomes 

 more variable under domestication, likewise as a direct effect of 



