516 THE EVOLUTION OF LIFE. 



need only be briefly indicated. It has already been shown 

 (First Principles, § 166), 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 respectively 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, incoherent 

 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 persistence with- 

 out progression. This apparent implication, however, is not 

 a real one. 



For though every environment on the Earth's surface 

 undergoes changes; and though usually the organisms which 

 each environment contains, cannot escape certain resulting 

 new influences; yet occasionally such new influences are 

 escaped, by the survival of species in the unchanged 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 unlikely 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 that to which the species has been habituated. 

 If, again, the circumstances of a species are modified by the 

 intrusion of some foreign kind of plant or animal, it follows 

 that since the intruders will probably not spread throughout 

 its whole habitat, the species will, in one or more localities, 

 remain unaffected by them. Especially among marine crea- 

 tures, must there frequently occur cases in which modifying 

 causes are continually eluded. Comparatively uniform as 



