122 EVOLUTION 



rence of a well-substantiated double-humped 

 curve — not the result of modificational 

 effects — may vividly bring home the fact 

 that the species is dividing into two sub- 

 species. Thus, by a statistical path, we are 

 brought face to face with the most vital of 

 all facts — revolution creatrice. 



The rapidly growing body of facts in re- 

 gard to variation is also confirming what 

 Darwin called the "correlation of varia- 

 tions." He pointed out that the whole 

 organization is so tied together during its 

 growth and development that, when slight 

 variations in any part occur, and are trans- 

 mitted, and are accumulated by natural 

 selection, other parts of the structure may 

 also undergo change, apparently irrespective 

 of any advantage. The whole framework is 

 so knit together that if one member suffer 

 change others suffer with it. 



The idea of correlation suggests that the 

 organism often changes as a unity in many 

 parts at once, and not like a machine that is 

 perfected piecemeal by the accumulation of 

 many little patents independent of each other. 

 Thus a variation important in the present 

 may bring in its train one that is destined to 

 be important in the future, and a variation 

 too small in itself to be of value may be 

 carried over the dead point into effective- 



