THE MUTATION HYPOTHESIS 63 



Species. On the other hand, Wallace, who advocated the 

 theory of evolution by natural selection at the same time 

 as Darwin, maintains, as Darwin did also most emphatically, 

 that large differences do not play the important part claimed 

 for them by de Vries and his followers. 



De Vries's " mutation theory " may be briefly stated as 

 follows : l Two kinds of variations occur normally among 

 living organisms; innumerable small variations occur con- 

 stantly ; they fluctuate round a specific mean much as the 

 shot-holes cluster round the bull's-eye of a target. Varia- 

 tion occurs in every direction, but the differences are small. 

 These are "fluctuating variations," and though they may 

 be selected up to a certain point, and a particular character 

 may be exaggerated in this way, such a character cannot be 

 permanently established, but will disappear as soon as the 

 selection ceases. On the other hand, large variations of 

 quite a different kind occur sometimes. Compared with 

 the "fluctuating variations" they are rare, and may not 

 occur in any given race for a long period of time. They 

 may, however, appear in considerable numbers in a particular 

 race at a particular period during its history. These are 

 " mutations" and de Vries holds that they are the only 

 kind of variation which is permanent. If the individual in 

 which a mutation has occurred produces young, and they 

 in their turn breed and perpetuate the race, this race will 

 form what he calls an elementary species. It will remain 

 permanently different from the stock in which the mutation 

 originally occurred. The way a new species is established 

 is by the selection, not of the fittest individual, but of the 

 fittest species. Mutations, that is, elementary species, which 

 are unfavourable are eliminated ; those which are favourable 

 survive. 



The first difficulty met with in de Vries's hypothesis is the 

 impossibility of distinguishing between a large fluctuating 

 variation and a small mutation. Apparently the only way 



1 De Vries, Hugo, Die Mutations -theoric, 2 vols., Leipzig ; vol. i., 1901 ; 

 vol. ii., 1903. 



