THE MULTIPLICATION OF EFFECTS. 459 



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 niultiplication 

 of effects which we concluded must have been instrumental 

 to the first, we see has in a great measure wrought 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 neAv 

 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 dis- 

 tinguishable from each other, were originally one race — 

 that the diffusion of one race into different climates and 

 conditions of existence has produced many altered 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 trans- 

 formed one original breed into numerous breeds, now be- 



?, it would doubtless have been otherwise expressed. Reference would 

 have been made to the process of " natural selection," as greatly facilitating 

 the differentiations described. As it is, however, I prefer to let the passage 

 stand in its original shape : partly because it seems to me that these succes- 

 sive changes of conditions would produce divergent varieties or species, apart 

 from the influence of " natural selection " (though in less numerous ways as 

 well as less rapidly) ; and partly because I conceive that in the absence of 

 these successive changes of conditions, " natural selection " would effect com- 

 paratively little. Let me add that though these positions are not enunciated 

 in The Origin of Species, yet a common friend gives me reason to think that 

 Mr. Darwin would coincide in them ; if he did not indeed consider them as 

 tacitly implied in his work. 



