DARWINISM DEFENDED. 155 



and, therefore, as long as conditions remain constant, the 

 tendency of selection 'must be, as Darwin clearly saw, cumu- 

 lative. He did not, however, apparently see that from this 

 cumulative tendency definite variability must arise out of 

 indefinite. 



"Selection in direct relation to climatic conditions is, there- 

 fore, of very minor importance, while selection among the 

 members of a species and all forms of inter-organismal 

 selection is of infinitely more importance, since it is this 

 interaction, produced by the offspring in different degrees 

 inheriting the advantages of both parents (both of whom 

 have survived on account of certain advantages), that leads 

 to the cumulative development and never-ending, struggle 

 for survival. 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 con- 

 ditions 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 in- 



*From Poulton's "Charles Darwin and the Theory of Natural 

 Selection" (Abstract of Darwin's letter to Professor Asa Gray). 



