116 ANALYSIS OF THE FOUR PRINCIPLES (CONTINUED). 
districts presenting the same environment.—In my paper reproduced in 
Appendix II of this volume I have emphasized the fact that it is true 
of a very wide range of species that any one species distributed in 
small sections in several isolated districts, presenting the same envi- 
ronment, will often use the environment in different ways, and so be 
subjected to different forms of selection. Selection thus determined 
by the relations in which the organism puts itself to the environment 
I call endonomic selection. 
We shall here consider in fuller detail the different conditions that 
may produce divergent forms of selection in isolated groups, exposed to 
the sameenvironment. Let us first consider cases in which the isolated 
groups are very small, and from a species with many variations through 
adaptations to a complex environment, and in which the new dis- 
tricts to which they are brought present the same environment as is 
found in the original home of the species. That the conditions may 
be clearly apprehended, let us suppose that we are considering a 
species of Hawaiian tree-snails on the southwest side of the main 
mountain range of Oahu, confined to the shady groves of a single val- 
ley, shut in on either side and at the head of the valley by high ridges 
covered with open brush, and at the mouth of the valley by grassy 
slopes that extend to the sea. This snail lives continuously on the 
trees, clinging to the trunks and large limbs of five or six species, and 
presents many variations of color and some divergences in acquired 
habitudes according to the species of tree on which it has lived. If for 
many generations a certain strain should live entirely on one species 
of trees (perhaps occupying a single grove which includes no other 
trees), it would present innate aptitudes for that kind of life, devel- 
oped by selection. Now, suppose that by some very rare accident a 
man, bearing a branch of a tree, unconsciously transports a single 
impregnated individual of this species of snail into the neighboring 
valley on one side, and within a few years a similar occurrence carries 
another individual of the same species, but occupying another kind 
of tree, into the valley on the other side. Each individual has occu- 
pied but one kind of tree for its whole life, and having formed habi- 
tudes strongly favoring the kind it has so far used, seeks and finds the 
same kind in the new district to which it has been brought. As there 
is no pressure of population in their new and previously unoccupied 
districts, the descendants of each remain for a hundred years or more 
in the grove in which the first comer settled down; and the two colo- 
nies have, perhaps for a hundred generations, been subjected to some- 
what divergent forms of selection; for the habits of feeding have been 
different, and there has been no crossing between those of different 
