1 1 6 ANALYSIS OF THE POUR PRINCIPLES (CONTINUED). 



districts presi nting tin same < nvironment. — 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 same environment. 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 grassv 

 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 bv 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 



