236 THE DEER OF AMERICA. 
overlap the upper part of the black so as to hide a portion of it. 
This condition usually occurs in the fall of the year and on 
young animals not more than three or four years old. The 
former condition is, to some extent at least, artificial. It results 
from those white hairs near the lower part of the tail becoming 
worn off so that they are shorter. This is rendered possible 
from the white hairs being open, spongy, non-elastic, and brittle, 
so that they are readily broken off by coming in contact with 
objects when the tail is moved by the animal. Those hairs near 
the upper end of the tail are less exposed to abrasion than those 
lower down and near the end. The black hairs composing the 
switch are different. They are quite solid, firm, and elastic, 
like those on the tail of the horse, and so can bear a great 
amount of friction without being broken off. They endure the 
violence, while the white hairs just above them become worn 
down short. The two extremes and an intermediate form are 
shown in the illustrations. Figs. 2, 3, 4, p. 234. 
Independent of this abrasion there is in most specimens a cer- 
tain degree of taper resulting from the fact that on them the 
white hairs are shorter on the lower than on the upper part of 
the tail. On aged specimens these white hairs are more brittle 
than on younger animals, hence they are worn shorter on them 
and so present a much more tapering form. 
Take a specimen about the first of August, when the old white 
hairs are nearly shed and the new ones are still short, and above 
the black tuft the tail is very small all the way up, while the 
black hairs which are constant, like those on the tail of the horse, 
remain conspicuous throughout the year. These black hairs are 
generally shorter and stand in a more radical position on the old 
specimens than on the young. On the young, that which is 
worn off of the black hairs is replaced by new growth which is 
more vigorous on the young than the old; on the latter the growth 
is tardy and does not fully repair the loss, so that on the young 
the black tuft appears longer and of less diameter than on the old. 
Ihave said that the under side of the tail of the Mule Deer. 
is naked down to near the end, like the horse’s tail. This is 
not absolutely so, for even the on young specimens a careful in- 
spection will reveal to the naked eyes a number of scattering 
very short fine hairs, more abundant towards the lower end of 
the naked space ; these grow somewhat larger as the animal 
grows older, but are always kept worn short, so that on old 
specimens the lower part of the naked portion exhibits a good 
