214 7 Colors of West Coast Mammals. [zon 
the extreme susceptibility of this plastic group to the influences of © 
environment, it is one of the most instructive and fascinating groups” 
among North American mammals. * * * * Probably a more 
striking illustration of evolution by environment cannot be cited.” 
Certain it is, that in the Eastern States, where the physiographic 
conditions are tolerably uniform, the species remains constant, while 
in the west (particularly in California) where there is much physio- 
graphic diversity, we find an extraordinary number of forms. Itis 
very doubtful, however, that the direct action of the environment _ 
has produced all, or even a large part of these changes. Aslongas 
a correlation can be established between light varieties with dry dis- 
tricts and dark varieties with rainy districts, it seems perfectly rea- 
sonable to suppose that the changes have been effected directly by 
the environment; but in a large number of cases such a correlation 
does not appear to exist; in which event the change must be attrib- 
uted to the indirect effect of environment. When the species first 
began to migrate from its primitive home it would find itself placed 
in many different situations. Wherever the character of the country — 
was materially different from its previous abode, a change in food — 
and to some extent in habits, occurred. This would have a tendency 
to modify the color in various ways. For example: if the animal 
lived in a cold or rainy climate it would be confined to its. burrow a 
comparatively large portion of the time, and any intensification of © 
the socialistic markings would be of great advantage. If, on the 
contrary, it lived in a warm, sunny region, it would be in the open 
air a large portion of the time, and protective coloration would be 
of greatest utility to the species, in which event the streaks would 
tend to become obscure or obliterated. This is merely an instance 
of the many ways in which environment might act indirectly. Hav- 
ing thus produced two races in adjacent regions, let us suppose that 
from some reason (persistence of bad weather, for example) one form 
migrates into the territory of the other. Difference in food, etc., 
granting the truth of the theory of physiological selection, have 
made the two races sterile when crossed, so that any accentuation 
of the slight differences already produced would favor the prosperity 
of both races. Accordingly, the markings originally of use as so- 
cialistic marks would become discriminative. Thus it is seen how 
complex the factors may be which operate in the production of even 
trivial changes. Simple explanations, in which a single factor | 
