36 Mutability and Individual Variation. 



variability. The chance variations were not therefore 

 the extreme variants of the ordinary variabiHty; they 

 were sporadic occurrences. Natural selection is on the 

 lookout for these, says Darwin, and seizes on them 

 ^'whenever and wherever opportunity offers.'''^ 



Darwin regarded these occasional deviations, these 

 mutations, as appearing from time to time and in a gen- 

 eral way conforming to definite laws as yet imperfectly 

 understood. According to these laws it could not hap- 

 pen that any considerable length of time should pass by 

 without the appearance of at least a few considerable 

 variations of this kind. To such variations would be 

 due the progress which the majority of living forms ex- 

 hibit in the course of the centuries. The longer the 

 time the better is the prospect of the appearance of favor- 

 able variations, 2 especially if these should only appear 

 very seldom.*^ They provide us with ''intermittent re- 

 sults."4 



Moreover Darwin went so far as to believe in a 

 certain periodicity. "Nascent species are more plastic," 

 that is to say produce more sports and have therefore a 

 better chance of splitting up into new species. Darwin 

 cites Naudin and Herbert as the authors of this view, 

 which they had derived from their comparative studies 

 of the forms occurring within certain groups of plants.^ 

 Schaafhausen^ mentions the unequal rate of the prog- 

 ress in different branches of the genealogical tree, in some 

 of them the changes taking place very quickly whilst in 

 others absolute stagnation seemed to be the rule during 

 long geological epochs. To produce a genuine new spe- 

 cies, a variety must from time to time, perhaps at long 



* Loc. a7., pp. 65, 66. '^ /&f J., pp. 82, 86. ^ /^/cf., pp. 85, 92 



* Ibid., p. 85. ^ Ibid., Hist. Sketch, p. xix. ^ Ibid., p. xx. 



