INTERNAL FACTORS. 428 



its members spread into other habitats. Those expansive 

 and repressive energies which set to each species a limit that 

 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 markedly contrasted with its average 

 circumstances. 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 fac- 

 tors, it brings us to the conclusion that there must be a pre- 

 vailing 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, 116), 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 

 that since " by the reactions of the differently modified parts 

 the incident force itself must be divided into differently 

 modified parts ; ' it follows that " each differentiated di- 

 vision of the aggregate thus becomes a centre from which 

 a differentiated 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 differentia- 

 tions/' And to this it was added, that in proportion aa 



