518 THE EVOLUTION OF LIFE. 



a common stock, is also liable to lose its original uniformity; 

 ^nd, in successive generations, to grow more pronounced in 

 its multifo^mit3^ 



These changes, which would go to but a comparatively 

 small extent were organisms exposed to constant external 

 conditions, are kept up by the continual changes in external 

 conditions, produced by astronomic, geologic, meteorologic, 

 and organic agencies: the average result being, that on 

 previous complications wrought by previous incident forces, 

 new complications are continually superposed by new inci- 

 dent forces. And hence simultaneously arises increasing 

 heterogeneity in the structures of individuals, in the struc- 

 tures of species, and in the structures of the Earth's Flora 

 and Fauna. 



But while, in very many or in most cases, the ever- 

 changing incidence of forces is ever adding to the complexity 

 of organisms, and to the complexity of the organic world as 

 a whole; it does this only where its action cannot be eluded. 

 And since, by migration, it is possible for a species to keep 

 itself under conditions that are tolerably constant, there 

 must be a proportion of cases in which greater heterogeneity 

 of structure is not to be expected. 



To show, however, that there must arise a certain average 

 tendency to the production of greater heterogeneity is not 

 sufficient. Aggregates might be rendered more heterogeneous 

 by changing incident forces, without having given to them 

 that kind of heterogeneity required for carrying on life. 

 Hence it remains now to inquire how the production and 

 maintenance of this kind of heterogeneity is insured. 



