THE VITAL FABRIC OF DESCENT 305 



the whole specific aggregation of interbreeding individuals. 

 The real and permanent advance is made in the main body of 

 the species, not among the stragglers from the flanks, nor by 

 the distraught captives of our cages, pastures and gardens. 



That the plant mutations which " come true to seed " are 

 often extremely uniform or constant, does not make it certain 

 that they are true species, but indicates, rather, the contrary, 

 since prosperous natural species show abundant individual 

 diversity. To give such "sports" formal descriptions and 

 Latin names does not prove that they represent genuine species 

 formed in the normal course of evolution ; it simply assumes 

 the identity of two biological conditions essentially distinct. 



The possibility that mutations, or even genetic variations, 

 may also be induced by new environmental conditions, as 

 believed by Darwin, is not excluded. But even in such cases 

 the environment would need to be regarded as furnishing the 

 occasion of the change, rather than as being the true, actuating 

 cause. Very diverse mutations, of the coffee plant, for ex- 

 ample, have been found to arise under the same environment, 

 and closely similar mutations under very different environ- 

 ments. 



The changes by which many organisms are able to accom 

 modate themselves to different conditions appear to be of little 

 or no direct significance for evolutionary purposes, though the 

 diversity manifested under the different conditions may serve 

 the same physiological purposes as other intraspecific differ 

 ences in connection with symbasic interbreeding. Evolution is 

 an integration of genetic variations, not of environmental in- 

 fluences. 



Segregation, or isolation, conduces to the formation of new 

 species by the subdivision of older groups, but it is not on that 

 account to be reckoned as a cause of evolution. Free inter- 

 breeding throughout the range of a species tends to keep the 

 characters uniform, but it does not tend to keep them stationary. 

 The characters remain relatively uniform because interbreeding 

 holds the members of the group well together on their evolu- 

 tionary pathway, not because progress is prevented by inter- 

 breeding. Free interbreeding '* swamps the incipient lines of 



