er organism. Taken in the mass, these divergent varieties
which have been caused by fresh physical conditions and habits of life,
will exhibit changes quite indefinite in kind and degree; and changes
that do not necessarily constitute an advance. Probably in most cases
the modified type will be neither more nor less heterogeneous than the
original one. In some cases the habits of life adopted being simpler
than before, a less heterogeneous structure will result: there will be a
retrogradation. But it _must_ now and then occur, that some division of
a species, falling into circumstances which give it rather more complex
experiences, and demand actions somewhat more involved, will have
certain of its organs further differentiated in proportionately small
degrees,--will become slightly more heterogeneous.

Thus, in the natural course of things, there will from time to time
arise an increased heterogeneity both of the Earth's flora and fauna,
and of individual races included in them. Omitting detailed
explanations, and allowing for the qualifications which cannot here be
specified, we think it is clear that geological mutations have all along
tended to complicate the forms of life, whether regarded separately or
collectively. The same causes which have led to the evolution of the
Earth's crust from the simple into the complex, have simultaneously led
to a parallel evolution of the Life upon its surface. In this case, as
in previous ones, we see that the transformation of the homogeneous into
the heterogeneous is consequent upon the universal principle, that every
active force produces more than one change.

The deduction here drawn from the established truths of geology and the
general laws of life, gains immensely in weight on finding it to be in
harmony with an induction drawn from direct experience. Just that
divergence of many races from one race, which we inferred must have been
continually occurring during geologic time, we know to have occurred
during the pre-historic and historic periods, in man and domestic
animals. And just that multiplication of effects which we concluded must
have produced the first, we see has produced the last. Single causes, as
famine, pressure of population, war, have periodically led to further
dispersions of mankind and of dependent creatures: each such dispersion
initiating new modifications, new varieties of type. Whether all the
human races be or be not derived from one stock, philology