DARWIN AND SPENCER ON CAUSES OF DIVERGENCE. 25 
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 theorv 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., 
Pp. 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. 109-115 (English edition). 
