6 INTRODUCTION. 
individuals with a special habit of feeding are transported to a new 
valley, it is not at all strange that they choose to pass by some plants 
used by many varieties of the species in the original home. More- 
over, their descendants may never regain the power to feed on as many 
kinds of plants; or, if on as many, probably not on the same kinds. 
Still further, when a distant valley has been reached, after a number 
of such transfers, each transfer being followed by a long history of 
habit building, without the influence of crossing with individuals of 
the original stock, is it at all strange that the habits have become 
widely divergent, and that they are the cause of divergent selection 
tending to establish the divergent habits in a more fixed form? 
In such a case as the one just described the new habit and the 
diversity of selection, with the diversity in the direct influence of the 
environment, can not be ascribed to any advantage over the old form 
resulting from the new habit; for competition with the old form has 
ceased for countless generations. Moreover, if the later history of 
the newer species has been in a new and unoccupied district, compe- 
tition with all allied forms has ceased; and the new habit is simply 
one of several forms of using the environment that are open to a new 
colony, unaffected by constant crossing with the old stock. 
Ill. May Not THE PREVENTION OF FREE CROSSING BE AN EXPLANATION? 
It will probably have already occurred to many of my readers that 
the wonderful limitation in the areas of distribution occupied by the 
separate species of Hawaiian snails is in some way connected with 
lack of powers and opportunities for migration; and now as we reflect 
that the same lack of migration would immeasurably increase the 
isolating effect of some rare occurrence by which a single individual 
is carried a mile or two beyond the home of the species, into a region 
of abundant food, the question naturally arises whether the isolation, 
which prevents all chance of crossing with the original stock, does not 
open the way for new habits, for new forms of selection, and accord- 
ingly for the transformation of the new colony into a new variety, 
and finally into a new species. 
We have here reached the idea of freedom from crossing with the 
old stock, or isolation, in the broader meaning which has been given 
to the term by writers on evolution since the days of Darwin. I be- 
lieve that no process of natural selection, or of sexual selection, or of 
any other form of selection, can transform one species into two or 
more species without the prevention of free crossing between the 
branches that are thus transformed. Isolation is, I believe, an essen- 
