INTERNAL FACTORS. 511 



perpetually oscillates from side to side of a certain mean, are, 

 as we lately saw, frequently changed by new combinations of 

 the external factors astronomic, geologic, meteorologic, and 

 organic. Hence there from time to time arise lines of di- 

 minished resistance, along which the species flows into new 

 localities. Such portions of the species as thus migrate, are 

 subject to circumstances unlike its previous average circum- 

 stances. And from multiformity of the circumstances, must 

 come multiformity of the species. 



Thus the law of the instability of the homogeneous has 

 here a three-fold corollary. As interpreted in connexion with 

 the ever-progressing, ever-complicating changes in external 

 factors, it involves the conclusion that there is a prevailing 

 tendency towards greater heterogeneity in all kinds of 

 organisms, considered both individually and in successive 

 generations; as well as in each assemblage of organisms con- 

 stituting a species ; and, by consequence, in each genus, order, 

 and class. 



155. When considering the causes of evolution in 

 general, we further saw (First Principles, 156), that the 

 multiplication of effects aids continually to increase that 

 heterogeneity into which homogeneity inevitably lapses. It 

 was pointed out that since "the several parts of an aggre- 

 gate are differently modified by any incident force ; " and 

 since " by the reactions of the differently modified parts the 

 incident force itself must be divided into differently modi- 

 fied parts ; " it follows that " each differentiated division 

 of the aggregate thus becomes a centre from which a differ- 

 entiated division of the original force is again diffused. And 

 since unlike forces must produce unlike results, each of these 

 differentiated forces must produce, throughout the aggregate, 

 a further series of differentiations." To this it was added 

 that, in proportion as the heterogeneity increases, the compli- 

 cations arising from this multiplication of effects grow more 

 marked; because the more strongly contrasted the parts of 



