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 makes it
clear that whole groups of races now easily distinguishable from each
other, were originally one race,--that the diffusion of one race into
different climates and conditions of existence, has produced many
modified forms of it. Similarly with domestic animals. Though in some
cases--as that of dogs--community of origin will perhaps be disputed,
yet in other cases--as that of the sheep or the cattle of our own
country--it will not be questioned that local differences of climate,
food, and treatment, have transformed one original breed into numerous
breeds now become so far distinct as to produce unstable hybrids.
Moreover, through the complication of effects flowing from single
causes, we here find, what we before inferred, not only an increase of
general heterogeneity, but also of special heterogeneity. While of the
divergent divisions and subdivisions of the human race many have
undergone changes not constituting an advance; while in some the type
may have degraded; in others it has become decidedly more heterogeneous.
The civilized European departs more widely from the vertebrate ar