ns which now begin to show themselves, we may
see coming into play that general law of the multiplication of effects
flowing from one cause, to which the increase of heterogeneity was
elsewhere ascribed.[9]

Confining our remarks, as we do, to the most general facts of
development, we think that some light is thus thrown on them. That the
unstable equilibrium of a homogeneous germ must be destroyed by the
unlike exposure of its several units to surrounding influences, is an _a
priori_ conclusion. And it seems also to be an _a priori_ conclusion,
that the several units thus differently acted upon, must either be
decomposed, or must undergo such modifications of nature as may enable
them to live in the respective circumstances they are thrown into: in
other words--_they must either die or become adapted to their
conditions_. Indeed, we might infer as much without going through the
foregoing train of reasoning. The superficial organic units (be they the
outer cells of a "mulberry-mass," or be they the outer molecules of an
individual cell) must assume the function which their position
necessitates; and assuming this function, must acquire such character as
performance of it involves. The layer of organic units lying in contact
with the yelk must be those through which the yelk is absorbed; and so
must be adapted to the absorbent office. On this condition only does the
process of organization appear possible. We might almost say that just
as some race of animals, which multiplies and spreads into divers
regions of the earth, becomes differentiated into several races through
the adaptation of each to its conditions of life; so, the originally
homogeneous population of cells arising in a fertilized germ-cell,
becomes divided into several populations of cells that grow unlike in
virtue of the unlikeness of their circumstances.

Moreover, it is to be remarked in further proof of our position, that it
finds its clearest and most abundant illustrations where the conditions
of the case are the simplest and most general--where the phenomena are
the least involved: we mean in the production of individual cells. The
structures which presently arise round nuclei in a blastema, and which
have in some way been determined by those nuclei as centres of
influence, evidently conform to the law; for the parts of the blastema
in contact with the nuclei are differently conditioned from the parts
not in contact with them. Again, the formation