NATURAL SELECTION 393 



Darwin came very near to this conception of definite variability when 

 he pointed out that "if a country were changing, the altered conditions 

 would tend to cause variation, not but what I believe most beings 

 vary at all times enough for selection to act on." Extermination 

 would expose the remainder to the "mutual action of a different set of 

 inhabitants, which I believe to be more important to the life of each 

 being than mere climate," and as "the same spot will support more 

 life if occupied by very diverse forms," it is evident that selection 

 will favour very great diversity of structure. 



Bearing in mind this cumulative action of selection it will follow 

 that under constant or relatively constant conditions the struggle for 

 successful living will become more and more selective in character, 

 even if the actual number of inhabitants remain more or less the same 

 as when the struggle first commenced. The selection of variations 

 will thus tend to pass through certain more or less ill-defined, but 

 nevertheless, real stages. In proportion as the struggle becomes 

 intense, either from the number or from the increasing adaptability 

 of the organisms, or both, certain major essential adaptations, which 

 were necessary for the climatic and other more or less comparatively 

 simple conditions, will be supplemented by minor auxiliary variations 

 which in the earlier stages would not have appeared. And still later, 

 as more and more rigorous conditions of life were imposed, the advan- 

 tage would tend to rest with those organisms which possessed highly 

 co-ordinated adaptations, since this would entail more rapid respon- 

 siveness to environment. 



As evolution advances from the unspecialised to the specialised, 

 and higher and higher forms of life come into being, with increasing 

 complexity and specialisation of parts entailing an increasingly delicate 

 adjustment of those parts to each other's needs, the relation of each 

 part to the whole organism becomes of more and more importance, 

 and it follows that selection must become more and more generalised 

 in its action. No single variation could be of service to any of the 

 higher forms of life unless it was in more or less complete harmony 

 with the whole tendency of the individual. The adjustment of parts 

 and their mutual interdependence make it essential for adaptation 

 that the relation of parts be preserved; consequently, correlated 

 minute favourable variations will tend to be more and more selected 

 as evolution passes from the unspecialised to the specialised forms of 

 life. This response of the whole organism should be still more delicate 

 in those forms of life that are continually subjecting themselves to 



