ignificance to the
differentiation of its alimentary canal into stomach and intestine. That
in the higher _Annulosa_, a single heart replaces the string of
rudimentary hearts constituting the dorsal blood-vessel in the lower
_Annulosa_, (reaching in one species to the number of one hundred and
sixty), is a truth as much needing to be comprised in the history of
evolution, as is the formation of a respiratory surface by a branched
expansion of the skin. A right conception of the genesis of a vertebral
column, includes not only the differentiations from which result the
_chorda dorsalis_ and the vertebral segments imbedded in it; but quite
as much it includes the coalescence of numerous vertebral processes with
their respective vertebral bodies. The changes in virtue of which
several things become one, demand recognition equally with those in
virtue of which one thing becomes several. Evidently, then, the current
statement which ascribes the developmental progress to differentiations
alone, is incomplete. Adequately to express the facts, we must say
that the transition from the homogeneous to the heterogeneous is carried
on by differentiations and accompanying integrations.

It may not be amiss here to ask--What is the meaning of these
integrations? The evidence seems to show that they are in some way
dependent on community of function. The eight segments which coalesce to
make the head of a centipede, jointly protect the cephalic ganglion, and
afford a solid fulcrum for the jaws, &c. The many bones which unite to
form a vertebral skull have like uses. In the consolidation of the
several pieces which constitute a mammalian pelvis, and in the
anchylosis of from ten to nineteen vertebrae in the sacrum of a bird, we
have kindred instances of the integration of parts which transfer the
weight of the body to the legs. The more or less extensive fusion of the
tibia with the fibula and the radius with the ulna in the ungulated
mammals, whose habits require only partial rotations of the limbs, is a
fact of like meaning. And all the instances lately given--the
concentration of ganglia, the replacement of many pulsating blood-sacs
by fewer and finally by one, the fusion of two uteri into a single
uterus--have the same implication. Whether, as in some cases, the
integration is merely a consequence of the growth which eventually
brings into contact adjacent parts performing similar duties; or
whether, as in other cases, there is an a