of a mammal, which proceeds without break from its first to
its last stage, and that of an insect, which is divided into
strongly-marked stages--egg, larva, pupa, imago. Nevertheless it is now
an established fact, that all organisms are evolved after one general
method. At the outset the germ of every plant or animal is relatively
homogeneous; and advance towards maturity is advance towards greater
heterogeneity. Each organized thing commences as an almost structureless
mass, and reaches its ultimate complexity by the establishment of
distinctions upon distinctions,--by the divergence of tissues from
tissues and organs from organs. Here, then, we have yet another
biological law of transcendent generality.

Having thus recognized the scope of Transcendental Physiology as
presented in its leading truths, we are prepared for the considerations
that are to follow.

* * * * *

And first, returning to the last of the great generalizations above
given, let us inquire more nearly how this change from the homogeneous
to the heterogeneous is carried on. Usually it is said to result from
successive differentiations. This, however, cannot be considered a
complete account of the process. During the evolution of an organism
there occur, not only separations of parts, but coalescences of parts.
There is not only segregation, but aggregation. The heart, at first a
simple pulsating blood-vessel, by and by twists upon itself and becomes
integrated. The bile-cells constituting the rudimentary liver, do not
merely diverge from the surface of the intestine in which they at first
form a simple layer; but they simultaneously consolidate into a definite
organ. And the gradual concentration seen in these and other cases is a
part of the developmental process--a part which, though more or less
recognized by Milne-Edwards and others, does not seem to have been
included as an essential element in it.

This progressive integration, manifest alike when tracing up the several
stages passed through by every embryo, and when ascending from the lower
organic forms to the higher, may be most conveniently studied under
several heads. Let us consider first what may be called _longitudinal
integration_.

The lower _Annulosa_--worms, myriapods, &c.--are characterized by the
great numbers of segments of which they respectively consist, reaching
in some cases to several hundreds; but as we advance to the higher
_Annulosa_--