ifferent parts; so, in this case too, difference of circumstances
is the primary cause of differentiation. Add to which, that as the
several changes undergone by the respective parts thus diversely acted
upon, are changes which do not destroy their vital activity, they must
be changes which bring that vital activity into subordination to the
incident forces--they must be adaptations; and the like must be in some
sense true of all the subsequent changes. Thus by deductive reasoning we
get some insight into the method of organization. However unable we are,
and probably ever shall be, to comprehend the way in which a germ is
made to take on the special form of its race, we may yet comprehend the
general principles which regulate its first modifications; and,
remembering the unity of plan so conspicuous throughout nature, we may
_suspect_ that these principles are in some way concerned in succeeding
modifications.

* * * * *

A controversy now going on among zoologists, opens yet another field for
the application of the deductive method. We believe that the question
whether there does or does not exist a _necessary correlation_ among the
several parts of an organism is determinable _a priori_.

Cuvier, who first asserted this necessary correlation, professed to base
his restorations of extinct animals upon it. Geoffroy St. Hilaire and
De Blainville, from different points of view, contested Cuvier's
hypothesis; and the discussion, which has much interest as bearing on
paleontology, has been recently revived under a somewhat modified form:
Professors Huxley and Owen being respectively the assailant and defender
of the hypothesis.

Cuvier says--"Comparative anatomy possesses a principle whose just
development is sufficient to dissipate all difficulties; it is that of
the correlation of forms in organized beings, by means of which every
kind of organized being might, strictly speaking, be recognized by a
fragment of any of its parts. Every organized being constitutes a whole,
a single and complete system, whose parts mutually correspond and concur
by their reciprocal reaction to the same definite end. None of these
parts can be changed without affecting the others; and consequently each
taken separately, indicates and gives all the rest." He then gives
illustrations: arguing that the carnivorous form of tooth necessitating
a certain action of the jaw, implies a particular form in its condyles;