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 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 complications 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 civilised European departs more widely from the vertebrate archetype than does the savage.  Thus, both the law and the cause of progress, which, from lack of evidence, can be but hypothetically substantiated in respect of the earlier forms of life on our globe, can be actually substantiated in respect of the latest forms.