I 1 8 ANALYSIS OF THE POUR PRINCIPIvES (CONTINUED) . 



case of the snails just considered some of them are brought to groves 

 on the northeast side of the mountain range, they will be exposed to a 

 somewhat greater rainfall, and will probably be subjected to some 

 change through the resulting selection. Again, suppose a colony is 

 planted on either side of the range, in a valley where but one kind of 

 shade trees is found, and this a kind that has never before been occu- 

 pied by the species. In both these cases we should have heteronomic 

 selection of the form that has been called natural selection. If any 

 of these snails should be discovered by man to be good for food or 

 medicine, and should be subjected to selection for the purpose of 

 improving the qualities sought, the result would be heteronomic 

 selection of the artificial form. 



Environal election corresponds with environal selection in the 

 general influences by which it is shaped ; but it differs in the results 

 produced, for it relates to the intensifying of habitudes and acquired 

 characters within the associating group. The higher the grade of 

 intelligence the more marked are the changes and divergences intro- 

 duced by acquired habitudes and characters, and accordingly, in such 

 cases, endonomic election becomes the leading factor by which some 

 new adjustment to the environment is developed into an established 

 method of sustaining life ; and if the inherited endowments are not in 

 complete accord with the new life, coincident selection carries the 

 adjustment to higher degrees; for variations favoring the conditions 

 imposed by the new tradition will have the advantage. Kxamples of 

 endonomic election preceding and introducing coincident selection 

 are seen in the tree-climbing rats mentioned above,* and in the cats 

 that have taken to wading and fishing, f Heteronomic election is 

 either natural or artificial. Artificial election is seen in dogs and other 

 domestic animals that have been subjected to training. Natural 

 election is seen in the case of the chimney swift, which, in a large meas- 

 ure, having lost the hollow trees in which it used to build its nests, 

 has been forced to find a substitute in the chimneys built by the 

 intruders who cut down its trees. The new habit is' undoubtedly 

 being reinforced by instincts gradually established by coincident 

 natural selection. 



2. The Methods of Environal Isolation. 



Endonomic isolation. — It is evident that, when varieties of the same 

 species of plant occupying the same areas are prevented from crossing 

 by flowering at different seasons, the process which I call seasonal 

 isolation is rightly classed as a form of endonomic isolation. The 



* See page loi. f See pages 67- 



