osition in one or more of the tissues. That is to say,
of the aggregate of similar organic units composing a germ, the group
going to the formation of a particular tissue, will take on the special
character which the adaptation of that tissue to new circumstances had
produced in the parents. We know this to be a general law of organic
modifications. Further, it is the _only_ law of organic modifications of
which we have any evidence.[10] It is not impossible then that it is the
universal law; comprehending not simply those minor modifications which
offspring inherit from recent ancestry, but comprehending also those
larger modifications distinctive of species, genus, order, class, which
they inherit from antecedent races of organisms. And thus it _may be_
that the law of adaptation is the sole law; presiding not only over the
differentiation of any race of organisms into several races, but also
over the differentiation of the race of organic units composing a germ,
into the many races of organic units composing an adult. So understood,
the process gone through by every unfolding organism will consist,
partly in the direct adaptation of its elements to their several
circumstances, and partly in the assumption of characters resulting from
analogous adaptations of the elements of all ancestral organisms.

But our argument does not commit us to any such far-reaching speculation
as this; which we introduce simply as suggested by it, not involved. All
we are here concerned to show, is, that the deductive method aids us in
interpreting some of the more general phenomena of development. That all
homogeneous aggregates are in unstable equilibrium is a universal truth,
from which is deducible the instability of every organic germ. From the
known sensitiveness of organic compounds to chemical, thermal, and other
disturbing forces, we further infer the _unusual_ instability of every
organic germ--a proneness far beyond that of other homogeneous
aggregates to lapse into a heterogeneous state. By the same line of
reasoning we are led to the additional inference, that the first
divisions into which a germ resolves itself, being severally in a state
of unstable equilibrium, are similarly prone to undergo further changes;
and so on continuously. Moreover, we have found it to be equally an _a
priori_ conclusion, that as, in all other cases, the loss of homogeneity
is due to the different degrees and kinds of force brought to bear on
the d