ther. These are species--Buffon adheres to the genetic
definition of species--and the species is a much more definite unit
than the genus, the order, the class, which are not divisions imposed
by us upon Nature. Species are definitely discontinuous,[33] and this
is the only discontinuity which Nature shows us. Buffon put his views
into practice in his _Histoire Naturelle_, where he describes species
after species, never uniting them into larger groups. We have seen,
however, how the facts forced upon him the conception of the "family."

Buffon was no morphologist. He left to Daubenton what one might call
the "dirty work" of his book, the dissection and minute description of
the animals treated.

But Buffon was a man of genius, and accordingly his ideas on
morphology are fresh and illuminating. Few naturalists have been so
free from the prejudices and traditions of their trade. He makes in
the _Discours sur la Nature des Animaux_[34] a distinction, which
Bichat and Cuvier later developed with much profit, between the
"animal" and the "vegetative" part of animals.[35] The vegetative or
organic functions go on continuously, even in sleep, and are performed
by the internal organs, of which the heart is the central one. The
active waking life of the animal, that part of its life which
distinguishes it from the plant, involves the external parts--the
sense-organs and the extremities. An animal is, as it were, made up of
a complex of organs performing the vegetative functions, assimilation,
growth, and reproduction, surrounded by an envelope formed by the
limbs, the sense-organs, the nerves and the brain, which is the centre
of this "envelope."[36] Animals may differ from one another enormously
in the external parts, particularly in the appendicular skeleton,
without showing any great difference in the plan and arrangement of
their internal organs. Quadrupeds, Cetacea, birds, amphibians and fish
are as unlike as possible in external form and in the shape of their
limbs; but they all resemble one another in their internal organs. Let
the internal organs change, however--the external parts will change
infinitely more, and you will get another animal, an animal of a
totally different nature. Thus an insect has a most singular internal
economy, and, in consequence, you find it is in every point different
from any vertebrate animal.

In this contrast, on the whole justified, between the importance of
variations in the "vegetativ