at after
a time, the length of which will depend on the nature of the bodies,
both will acquire the same temperature. In very warm climates the bodies
of animals derive from the sun, and from the heated bodies surrounding
them, more heat than they give in return; and were it not for their
internal cooling apparatus, which I have described, the heat so absorbed
would prove fatal. In every climate, on the contrary, where the
temperature is lower than 98 deg., or "blood heat," the bodies of animals
lose more heat by radiation than they receive by the same means. The
philosophy of the _clothing_ of men and the _sheltering_ of the lower
animals is now evident. It is not only necessary that heat should be
developed within the body, but also that its wasteful expenditure should
be prevented. The latter is effected by interposing between the warm
body and the cold air some substances (such as fur or wool) which do not
readily permit the transmission of heat--_non-conductors_ as they are
termed. The close down of the eider duck is destined to protect its
bosom from the chilling influence of the icy waters of the North Polar
Sea, and the quadrupeds of the dreary Arctic Circle are sheltered by
thick fur coverings from the piercing blasts of its long winter.

_Fat Equivalents._--Whilst it is quite certain that neither nerves nor
muscles can be elaborated exclusively out of fat, starch, sugar, or any
other non-nitrogenous substance, it is almost equally clear that fat may
be formed out of nitrogenous tissue. The quantity of fat, however, which
is produced in the animal mechanism, from purely nitrogenous food
appears to be relatively very small. No animal is capable of subsisting
solely on muscle-forming materials, no matter how abundantly supplied.
The food of the Carnivora contains a large proportion of fat, and the
nutriment of the Herbivora is largely made up of starch and other
fat-formers. Dogs, geese, and other animals fed exclusively upon albumen
or white of egg rapidly decreased in weight, and after presenting all
the symptoms of starvation, died in three or four weeks.[8] The fat of
the bodies of the Carnivora is almost entirely formed--and probably with
little if any alteration--from the fatty constituents of their food.
Herbivorous animals, on the contrary, derive nearly all their fat from
starch, sugar, gum, cellulose, and other non-nitrogenous, but not fatty,
materials.

Although starch is convertible into fat, it is n