270 THE INDUCTIONS OF BIOLOGY. 



to affect their constitutions profoundly, and to modify some- 

 what the physiological units thrown off in their reproductive 

 cells, the divergences produced by these in offspring, will be 

 of diverse kinds. And the original homogeneity of constitu- 

 tion having been thus destroyed, variation may go on with 

 increasing facility. There will result a heterogeneous mix- 

 ture of modifications of structure, caused by modifications of 

 function ; and of still more numerous correlated modifica- 

 tions, indirectly so caused. By natural selection of the most 

 divergent forms, the unlikenesses of parents will grow 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, % 126). 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- 



