ch adaptation for the good of the
species will be considered in the next chapter.

The first and most important struggle any animal has to enter is the
never-ending battle for its food. Occasionally there is a similar
straining after the air it breathes. But ordinarily air is
sufficiently abundant, except to animals living in the water, where
the supply is always more or less restricted and easily becomes
exhausted. But food is the constant need of every organism, and most
creatures die for lack of it. In this struggle the animal is pitted
against those of his own kind, rather than against those of other
species. Even his brother is his enemy, for he desires the same food.
In many a nest of birdlings one of them fails to reach its development
simply because the parent either is unable to find or it cannot carry
enough food to satisfy all the hungry mouths in the same nest. Before
the nestlings are ready to take their place in the struggle for life
outside and hunt their own living, one or more of them has succumbed.

After the battle for food comes the struggle for shelter. For most
animals there is no such thing as shelter. They are exposed to the
inclemencies of the weather and to the depredations of their enemies
without the means of retiring into any situation which might protect
them. In the higher animals, especially when they are warmer blooded
and their bodies must be kept at a higher temperature, some form of
covering has come to be almost universal.

Though comparatively few animals are prepared to seek shelter from the
cold, all of them have enemies against whom they must battle. These
foes may wish to eat them or may simply wish to get them out of the
way. In either event this struggle is so persistent and so keen that
after starvation it is probably the source of the largest loss to the
animal kingdom.

Considering first the feeding habits of animals, we find they are
exceedingly varied. Some creatures simply engulf other and more minute
animals, often only microscopic in size, in such quantities as to
satisfy their hunger. Others, feeding upon larger plants or animals,
must have some means of breaking off particles of this food; still
others confine themselves entirely to nutritious fluids, and must have
organs adapted to this particular type of food.

Insects are so common that anyone, who cares to, may easily verify
what is here described. It will take nothing but a clear observant eye
and a little