e" and variations in the "animal" parts,
one may see without doing violence to Buffon's thought, an indication
of the difference between homology and analogy. It is usually in the
external parts, in the organs by which the animal adapts itself to its
environment, that one meets with the greatest number of analogical
resemblances. This contrast of vegetative and animal parts and their
relative importance for the discovery of affinities was at any rate a
considerable step towards an analysis of the concept of unity of plan.

To Xavier Bichat (1771-1802) belongs the credit of working out in
detail the distinction drawn by Aristotle and Buffon between the
animal and the vegetative functions. Bichat was not a comparative
anatomist; his interest lay in human anatomy, normal and pathological.
So his views are drawn chiefly from the consideration of human
structure.

He classifies functions into those relating to the individual and
those relating to the species. The functions pertaining to the
individual may be divided into those of the animal and those of the
organic life.[37] "I call _animal life_ that order of functions which
connects us with surrounding bodies; signifying thereby that this
order belongs only to animals" (p. lxxviii.). Its organs are the
afferent and efferent nerves, the brain, the sense-organs and the
voluntary muscles; the brain is its central organ. "Digestion,
circulation, respiration, exhalation, absorption, secretion,
nutrition, calorification, or production of animal heat, compose
organic life, whose principal and central organ is the heart" (p.
lxxix.).

The contrast of the animal and the organic life runs through all
Bichat's work; it receives classical expression in his _Recherches
Physiologiques sur la Vie et la Mort_ (1800). The plant and the animal
stand for two different modes of living. The plant lives within
itself, and has with the external world only relations of nutrition;
the animal adds to this organic life a life of active relation with
surrounding things (3rd ed., 1805, p. 2). "One might almost say that
the plant is the framework, the foundation of the animal, and that to
form the animal it sufficed to cover this foundation with a system of
organs fitted to establish relations with the world outside. It
follows that the functions of the animal form two quite distinct
classes. One class consists in a continual succession of assimilation
and excretion; through these functions the ani