428 THE EVOLUTION OF LIFE. 



How ill each, species, considered as an aggregate of indi- 

 viduals, there must arise stronger and stronger contrasts 

 between those divergent varieties which result from the 

 instability of the homogeneous and the multiplication of 

 effects, needs only be briefly indicated. It has already 

 been shown (First Principles, § 126), that in conformity to 

 the universal law that mixed units are segregated by like 

 incident forces, there are produced increasingly-definite 

 distinctions among varieties, wherever there occur definitely- 

 distinguished sets of conditions to which the varieties are re- 

 spectively subject. 



§ 157. Probably in the minds of some, the reading of this 

 chapter has been accompanied by a running commentary, to 

 the effect that the argument proves too much. The apparent 

 implication is, that the passage from an indefinite, incohe- 

 rent homogeneity to a definite, coherent heterogeneity in 

 organic aggregates, must have been going on universally ; 

 whereas we find that in many cases there has been persist- 

 ence without progression. This apparent implication, how- 

 sver, is not a real one. 



For though every environment on the Earth's surface 

 an der goes changes ; and though usually the organisms 

 which each environment contains, cannot escape certain 

 resulting new influences ; yet occasionally such new in- 

 fluences are escaped, by the survival of species in the un- 

 changed parts of their habitats, or by their spread into 

 neighbouring habitats which the change has rendered like 

 their original habitats, or by both. Any alteration in the 

 temperature of a climate or its degree of humidity, is un- 

 likely to affect simultaneously the whole area occupied by a 

 species ; and further, it can scarcely fail to happen that the 

 addition or subtraction of heat or moisture, will give to a 

 part of some adjacent area, a climate like to that to which 

 the species has been habituated. If, again, the circumstances 

 of a species are modified by the intrusion of some foreign 



