INCEEASING DIVEEGENCE OF THE ANIMAL EACE8. 4:9 



time to time arise an increased heterogeneity both of the 

 Earth's flora and famia, 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 sepa- 

 rately 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 princi- 

 ple, 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 pe- 

 riods, in man and domestic animals. And just that multi- 

 plication of effects which we concluded must have pro- 

 duced the first, we see has produced the last. Single 

 causes, as famine, pressure of population, war, have period- 

 ically led to further dispersions of mankind and of depend- 

 ent creatures : each such dispersion initiating new modifi 

 cations, 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 distinguisha- 

 ble from each other, were originally one race, — that the 

 diff'usion 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 

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