206 The Origin of Species by Miitaiioji. 



conditions of life" are the chief causes of this transmu- 

 tation, and Darwin cannot have imagined the environ- 

 ment to have been perpetually changing. Moreover Dar- 

 win often refers to the fact that a plant exhibits little 

 variability during the first few years after it has been 

 brought into cultivation but after 3 or 5 years begins to 

 give rise to new forms. Even if the explanation of this 

 phenomenon should turn out to be different from that 

 given by Darwin, the fact that he insists so strongly 

 upon it shows at any rate that the idea of periods of 

 greater and of less mutability was present in his mind.-^ 

 As the cause of these periods, Darwin believed that ex- 

 ternal influences must act for many generations before 

 they can induce any change of this kind. 



But if mutability is a periodical phenomenon we get 

 round the difficulty of having to suppose that mutations 

 should appear equall}^ at all times; and we are also in a 

 position to account for the apparent periodicity in evo- 

 lution. The existence of long intervals of time during 

 which characters remain unaltered is, at any rate in the 

 case of a great many species, a matter of tolerable cer- 

 tainty. The frequent, although not universal, existence of 

 the same elementary species in localities which have been 

 separated for centuries points decisively in this direction. 



Moritz Wagner's famous theory of migration is 

 based on the same fundamental idea.- We have no rea- 

 son to expect mutability so long as its external causes 

 are absent. So long, that is to say, as the climatic, phys- 



of the constant forms which actually exist." Correspondenzhlatt d. d. 

 Ges. f. Anthropologie, Vol. 31, No. i, p. 3, Jan. 1900. 



^ "I do believe that natuval selection will generally act very 

 slozvh', only at long intervals of time.'" (Darwin, Origin, 6th ed., 

 p. 850 



"Wagner^ Das Migrationsgesets der Organismen. 



