Mollusca of Sandwich Isles 
to the amount of change which can be effected 
by the accumulation of fluctuating variations ; 
but, as we have already seen (on p. 70), there 
is a very definite limit and this limit is quickly 
reached. 
Thus the arguments of Romanes and Gulick 
are fundamentally unsound. 
But the fact remains, and has to be accounted 
for, that, as a general rule, when two portions of 
a species are separated, so that they are pre- 
vented from interbreeding, they begin to diverge 
in character, and the longer they remain thus 
separated the greater becomes that divergence. 
This is an observed fact which cannot be 
gainsaid. 
It was the observance of this fact which led 
Gulick to insist with such emphasis on the im- 
portance of geographical isolation as a factor in 
evolution. He discovered that the land mollusca 
of the Sandwich Islands fall into a great number 
of varieties. 
These islands are very hilly, and Gulick found 
that each of the varieties is confined not merely 
to one island, but to one valley.“ More- 
over,” writes Romanes, on p. 16 of Darwin 
and after Darwin, “on tracing this fauna from 
valley to valley, it is apparent that a slight 
variation in the occupants of valley 2, as com- 
pared with those of the adjacent valley 1, 
becomes more pronounced in the next, valley 3, 
375 
