great class of specialization is seen in the changes of habit
that provide the animal with shelter. The home seems so necessary a
part of human life that it is almost impossible to think of an animal
having nothing that in the faintest degree could be called a home. We
at least expect it to have some sheltered place in which it passes
most of its time and to which it returns after its wanderings. The
great majority of all animals have no such home. The place in which we
find them to-day may not be the place in which they will be to-morrow.
All places are alike to them. The ordinary conduct of their daily life
drives them about in the search for food. Their attempt to escape from
their enemies leads them each day into new situations, and they may,
and probably do, have no power to recognize the old location if they
return to it. When we come to the backboned animals there is a little
more tendency to a stationary location. The sun fish may frequent the
same reach of the stream, the trout may haunt the same pool, year
after year, but a great majority of fishes doubtless move
indiscriminately up and down the stream or about the lake or ocean and
are not found two successive days in the same place. The same may be
said of frogs. For a time a particular frog may have a fondness for a
special bend in the stream, but it is only a temporary fondness, I
believe.

Our own need for shelter is the prime motive in leading us to build a
home, and this necessity arises first of all because of our warm
blood. What we are accustomed to call cold-blooded animals are not
truly so. Their blood holds practically the temperature of their
surroundings. As the air or the water in which they live grows warmer
or colder the bodies of these creatures alter with it. Consequently
they are active when the temperature is high and grow more sluggish as
the thermometer falls. When the day grows distinctly cold the animals
may go practically dormant.

Only the birds and mammals have warm blood, and of these the birds are
distinctly the warmer. Whereas the temperature of the mammals runs
from about ninety-eight to a hundred degrees Fahrenheit, that of
birds lies somewhere between one hundred and five degrees and a
hundred and ten. Creatures which are warmer than their surroundings
must have some protection against chilling. Accordingly both mammals
and birds have clothing. In the case of mammals the covering is fur,
in the case of birds feathers. In so