INTENSITE SEOEEGATION. 349 



of the origin of varieties *. Variations are deviations from the 

 average, but varieties are groups of individuals ia which the 

 averages differ, and in which the iuheritable 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 positions without 

 suffering disadvantage. Divergence in these initial stages has 

 seemed to many to be an obscurer problem than the advancing 

 usefulness which sometimes entirely remodels an organ. For, as 

 Prof Le Conte has said, " Natural selection does not make an 

 organ useful, but only more useful." 



I believe that 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 JEvolution does not necessarily defend on either 

 change in or change of the environment. 



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 (that is geographical separa- 

 tion), 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 

 correspond remain constant" f. "There must be maintained a 



* See ' Evolution and its Relations to Religious Tliouglit,' by Joseph Le 

 Oonte, publislied by Appleton & Co., page 252. 



i' Tliougli 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. 10L(-115 (English edition). 



