stability, and are invariable only temporarily."

"Yet, to facilitate the study and knowledge of so many different
organisms it is useful to give the name of _species_ to every
similar collection of similar individuals which are perpetuated by
heredity (_generation_) in the same condition, so long as the
circumstances of their situation do not change enough to render
variable their habits, character, and form."

He then discusses fossil species in the way already described in
Chapter III. (p. 75).

The subject of the checks upon over-population by the smaller and weaker
animals, or the struggle for existence, is thus discussed in
Chapter IV.:

"Owing to the extreme multiplication of the small species, and
especially of the most imperfect animals, the multiplicity of
individuals might be prejudicial to the preservation of the species,
to that of the progress acquired in the improvement of the
organization--in a word, to the general order, if nature had not
taken precautions to keep this multiplication within due limits over
which she would never pass.

"Animals devour one another, except those which live only on plants;
but the latter are exposed to being devoured by the carnivorous
animals.

"We know that it is the strongest and the best armed which devour
the weaker, and that the larger kinds devour the smaller.
Nevertheless, the individuals of a single species rarely devour each
other: they war upon other races.[182]

"The multiplication of the small species of animals is so
considerable, and the renewals of their generations are so prompt,
that these small species would render the earth uninhabitable to the
others if nature had not set a limit to their prodigious
multiplication. But since they serve as prey for a multitude of
other animals, as the length of their life is very limited, and as
the lowering of the temperature kills them, their numbers are always
maintained in proper proportions for the preservation of their races
and that of others.

"As to the larger and stronger animals, they would be too dominant
and injure the preservation of other races if they should multiply
in too great proportions. But their races devouring each other, they
would only multiply slowly and in a small number at a time; this
would maintain in this respect the kind of equilibrium which should
exist.

"Finally, only man, considered separately from al