IN ORGANIC EVOLUTION 91 



everywhere and inevitably connected with the 

 course of descent of any species, variation, hered- 

 ity, selection, and isolation must appear; the first 

 two innate, part of the definition of organic life, 

 the last two extrinsic, arising from the necessities 

 of environment, and not one of these can find 

 leverage without the presence of each of the 

 others. Isolation as the factor longest over- 

 looked, though to the field naturahst the most 

 conspicuous of the four, must be advanced to the 

 post of honor beside the others, not instead of 

 any of them. 



