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 



