DARWIN AND SPENCBR ON CAUSES OP DIVERGENCE. 215 



are groups of individuals in which the averages differ, and in which 

 the inheritable characters differ. Still further, it is usually admitted 

 that the divergences presented by varieties are not always essential to 

 the well-being of the forms that possess them, and that in many cases 

 the forms that are confined to separate localities might exchange posi- 

 tions without suffering disadvantage. Divergence in these initial 

 stages has seemed to many to be an obscurer problem than the ad- 

 vancing usefulness which sometimes entirely remodels an organ. For 

 as Professor Le Conte has said, ' ' Natural selection does not make an 

 organ useful, but only more useful." 



I believe the theory of divergent evolution, presented in this and 

 the preceding paper, is applicable to the formation of divergences 

 during the stage when some of the differences, if not all, bring neither 

 advantage nor disadvantage to those that possess them. Whatever 

 we call these divergent forms, can we give any explanation of the 

 causes that have produced them ? 



(2) Divergent evolution does not necessarily depend on diverse envi- 

 ronments. In other words, it does not necessarily depend on change in 

 the conditions surrounding the organism, or on the organism being 

 brought into a district presenting a different set of conditions. 



Darwin maintains that isolation (by which he designates geo- 

 graphical separation), without any differences in the surrounding 

 organisms or in the physical conditions, presents no occasion for 

 divergence of character. He says, "If a number of species, after 

 having long competed with each other in their old home, were to 

 migrate in a body into a new and afterwards isolated country, they 

 would be little liable to modification." (Origin of Species, 6th ed., 



P- 319-) 



Spencer expresses the same idea by saying that "Vital actions 

 remain constant so long as the external actions to which they corre- 

 spond remain constant."* There must be maintained a tolerably 

 uniform species so long as there continues a tolerably uniform set of 

 conditions in which it may exist." (See Spencer's Principles of 

 Biology, sections 91, 156, 169, 170.) In other words, divergence of 

 character in the descendants of one stock occupying different districts 

 does not arise except as it is preceded by difference in the physical 

 conditions, or in the surrounding organisms, of the different districts. 

 After molding this thought in many forms, Spencer makes it the funda- 

 mental principle on which he builds not a small portion of his philos- 



* Though apparently opposed to his theory of "the production of certain local 

 forms by amixia," this same idea is found in Weismann's "Studies in the Theory 

 of Descent," pp. 109-1 15 (English edition). 



