334: THE INDUCTIONS OF BIOLOGY. 



thrown off in their reproductive cells, the divergences pro- 

 duced by these in offspring will be of divers kinds. And 

 the original homogeneity of constitution having been thus 

 destroyed, variation may go on with increasing facility. 

 There will result a heterogeneous mixture of modifications of 

 structure caused by modifications of function; and of still 

 more numerous correlated modifications, indirectly so caused. 

 By natural selection of the most divergent forms, the unlike- 

 nesses of parents will be rendered more marked, and the 

 limits of variation wider. Until at length the divergences of 

 constitutions and modes of life, become great enough to lead 

 to segregation of the varieties. 



91. That variations must occur, and that they must ever 

 tend, both directly and indirectly, towards adaptive modifica- 

 tions, are conclusions deducible from first principles; apart 

 from any detailed interpretations like the above. That the 

 state of homogeneity is an unstable state we have found to 

 be a universal truth. Each species must pass from the uni- 

 form into the more or less multiform, unless the incidence of 

 external forces is exactly the same for all its members, which 

 it never can be. Through the process of differentiation and 

 integration, which of necessity brings together, or keeps to- 

 gether, like individuals, and separates unlike ones from them, 

 there must nevertheless be maintained a tolerably uniform 

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

 conditions in which it may exist. But if the conditions 

 change, either absolutely by some disturbance of the habitat 

 or relatively by spread of the species into other habitats, then 

 the divergent individuals that result must be segregated 

 by the divergent sets of conditions into distinct varieties 

 (First Principles, 166). When, instead of contemplating 

 a species in the aggregate, we confine our attention to a 

 single member and its descendants, we see it to be a corollary 

 from the general law of equilibration that the moving equili- 

 brium constituted by the vital actions in each member of 



