GENESIS, HETJEDITY, AND VARIATION. 291 



considerable functional differences, have entailed decided dif- 

 ferences among the physiological units ; and when the differ- 

 ent physiological units, differently mingled in every individual, 

 come to be variously segregated and variously combined. 



Did space permit, it might be shown that this hypothesis 

 is a ke^^ to many further facts— to the fact that mixed races 

 tire comparatively plastic under new conditions ; to the fact, 

 that pure races show predominant influences when crossed 

 with mixed races ; to the fact that while mixed breeds are 

 often of larger growth, pure breeds are the more hardy — ■ 

 have functions less-easily thrown out of balance. But with- 

 out further argument, it will, I think, be admitted, that the 

 power of this hypothesis to explain so many phenomena, and 

 to bring under a common bond phenomena that seem so little 

 allied, is strong evidence of its truth. And such evidence 

 gains greatly in strength on observing that this hypothesis 

 brings the facts of Genesis, Heredity, and Variation into har- 

 mony with first principles. When we see that these plastic 

 physiological units, which we find ourselves obliged to assume, 

 are just such more integrated, more heterogeneous, more un- 

 stable, and more multiform atoms, as would result from con- 

 tinuance of the steps through which organic matter is reached — - 

 when we see that the differentiations of them assumed to oc- 

 cur in differently- conditioned aggregates, and the equilibra- 

 tions of them assumed to occur in aggregates which maintain 

 constant conditions, are but corollaries from those universal 

 principles implied by the persistence of force — when we see 

 that the maintenance of life in the successive generations of a 

 species, becomes a consequence of the continual incidence of 

 new forces on the species, to replace the forces that are ever 

 being rhythmically equilibrated in the propagation of the 

 species — and when we thus seer that these apparently-excep- 

 tional phenomena displayed in the multiplication of organic 

 beings, fall into their places as results of the general laws of 

 Evolution; we have weighty reasons for entertaining the 

 hypothesis which affords us this interpretation. 



