of the function
as much as its expression is to be regarded as a gradual process. In
Adaptation, the closest connection between the function and the
structure of an organ is thus indicated. Physiological functions govern,
in a certain sense, structure; and so far what is morphological is
subordinated to what is physiological" (_Elements_, pp. 8-9). Gegenbaur
recognised also that morphological differentiation depended largely on
the physiological division of labour (_Grundzuege_, p. 49).

It is clear that Gegenbaur realised vividly the importance of function,
and in this respect, as in others, he is far beyond Haeckel. The same
thing comes out markedly in his treatment of correlation. Haeckel had no
slightest feeling for the true meaning of correlation. For him, as for
Darwin, it reduced itself to a law of correlative variation, according
to which "actual adaptation not only changes those parts of the organism
which are directly affected by its influence, but other parts also, not
directly affected by it."[384] Such "correlative adaptation" was due to
nutrition being a "connected, centralised activity."

Gegenbaur, on the contrary, had a firm grasp of the Cuvierian
conception, and expressed it in unmistakable terms. "As indeed follows
from the conception of life as the harmonious expression of a sum of
phenomena rigorously determining one another, no activity of an organ
can in reality be thought of as existing for itself. Each kind of
function (_Verrichtung_) presupposes a series of other functions, and
accordingly every organ must possess close relations with, and be
dependent on, all the others" (_Grundzuege_, p. 71). The organism must be
regarded as an individual whole which is as much conditioned by its
parts as one part is conditioned by the others. For an understanding of
correlation a knowledge of functions, and of the functional relations of
the organism to its environment, is clearly indispensable.

Gegenbaur's morphological system was out-and-out evolutionary. "The most
important part of the business of comparative anatomy," in Gegenbaur's
eyes, "is to find indications of genetic connection in the organisation
of the animal body" (_Elements_, p. 67).

The most important clue to discovering this genetic connection is of
course that given by homology; it is indeed the main principle of
evolutionary morphology that what is common in organisation is due to
common descent, what is divergent is due to adaptation