The hair differs in the way of thickness and fineness, and of
length, according to the locality of the part in which it is found,
and according to the quality of skin or hide on which it grows. For,
as a general rule, the thicker the hide, the harder and the thicker is
the hair; and the hair is inclined to grow in abundance and to a great
length in localities of the bodies hollow and moist, if the localities
be fitted for the growth of hair at all. The facts are similar in
the case of animals whether coated with scales or with tessellates.
With soft-haired animals the hair gets harder with good feeding, and
with hard-haired or bristly animals it gets softer and scantier from
the same cause. Hair differs in quality also according to the relative
heat or warmth of the locality: just as the hair in man is hard in
warm places and soft in cold ones. Again, straight hair is inclined to
be soft, and curly hair to be bristly.