Effects of External Conditions 151 
Passing to the question of the adaptiveness of these experiment- 
ally produced modifications, that of the hair would surely seem 
to fall within this category. A complementary physiological 
explanation for the change would doubtless be likewise possible, 
had we a sufficient knowledge of the various processes concerned. 
The shrinkage or “drawing in” of the peripheral parts under the 
influence of a cold climate might also be regarded as adaptive, 
for the reduction of these thinly clothed surfaces would diminish, 
at least theoretically, the radiation of heat from the body. Here 
again a simple physiological explanation is likewise possible. We 
might either appeal to the effect upon the peripheral circulation 
(as does Allen) or to the direct influence of temperature upon the 
protoplasm of the growing parts. In the case of the feet of the 
mice in the above experiments, the greater activity of the young 
animals in the warm room, and the greater consequent exercise 
of the limbs, may possibly have played some part in bringing about 
the difference. 
To what degree the modifications which I have described have 
been due to temperature and to what degree they have been due to 
humidity is not clear under the conditions of the experiments. 
As has been stated, the two have varied inversely. Allen and 
Coues seem to regard such differences, when presented by mam- 
mals in nature, as due chiefly to the temperature factor. Never- 
theless, the former writer tells us, speaking of hares, that “there 
is also a marked tendency to an enlargement of the ears in propor- 
tion to the aridity of thehabitat.. . .. . . Inthis connection, 
also, attention may be called to the fact that all of the long-eared 
species of American hares are found exclusively over the niost arid 
portions ofthe continent. ©... .””.< And it may be added 
that the color of the pelage of mammals and that of the plumage 
of birds is well known to vary with the hygrometric conditions. 
In many species of birds the degree of pigmentation is said to be 
a function of the mean humidity of the habitat. “Tower, indeed, 
regards the humidity as being much more important than the 
temperature in the production of color changes in beetles. Until, 
41 Monographs of N. A. Rodentia, p. 272. 
