Effects of External Conditions 149 
and Allen cited many specific instances of this fact for mice, hares, 
squirrels and other rodents. Moreover, obvious seasonal changes 
are to be observed in some species. Speaking of the squirrel, 
Sciurus hudsonius, var. hudsonius, Allen says: “In summer the 
soles of the feet are naked, often wholly so to the heel; in winter 
they are wholly thickly furred, only the tubercles at the base of the 
toes being naked. ‘The general pelage is also much fuller, longer 
and softer in winter than in summer.” 
It may well be that the change in the quantity of hair which 
appears to have been produced in the white mice during the exper- 
iments above described was comparable to these seasonal changes, 
i. e., that the results were purely temporary, and would have dis- 
appeared with a cessation of the conditions employed.* Indeed, 
since the life of an individual hair is comparatively brief, it would 
be necessary to effect some permanent change in the physiological 
activity of the hair follicles, in order that differences such as these 
should endure. Whether or not these effects are permanent, it 
has been believed by many that various changes in the character 
of the hair coat occur in domestic animals as the result of trans- 
ferrence to an unaccustomed habitat. Darwin, indeed, tells 
us* that “Great heat, however, seems to act directly on the fleece; 
several accounts have been published of the changes which sheep 
imported from Europe undergo in the West Indies. Dr. Nichol- 
son of Antigua informs me that, after the third generation, the 
wool disappears from the whole body, except over the loins; and 
the animal then appears like a goat with a dirty door-mat on its 
back.” And again:® “It has been asserted on good authority 
[Isidore Geoffroy St. Hilaire] that horses kept during several years 
in the deep coal-mines of Belgium become covered with velvety 
hair, almost like that on the mole.’ The “classical’’ Porto 
Santo rabbit may be cited as another and perhaps more authentic 
instance of the modification of mammals through changed cli- 
8 Monographs of N. A. Rodentia, p. 675. 
44Tt is uncertain, to be sure, in how far the season changes of the hair coat of mammals are direct 
responses to climatic conditions. 
45 Variation of Animals and Plants under Domestication, vol. i, p. 124. 
4 Op. cit., vol. ii, p. 336. 
