Darwin's Selection Theory. Z2> 



etc. ; these cases are instances of retrogressive mutability 

 and have no signification for the elucidation of the main 

 lines of descent. 



Apart from this quite definite group of modifications 

 hy loss, single variations seem to be presented by all char- 

 acters, to proceed in every direction and to be apparently 

 without limit. To sum up, individual differences are al- 

 ways present, occur in every direction and in every char- 

 acter, but are limited and conform to definite laws. Single 

 variations, on the other hand, are sporadic phenomena, 

 appearing only from time to time, and suddenly changing 



Fig. 6. Fragaria alpina, IMonthly Strawberry. 

 (Fraisier des quatre saisons.) 



the forms of life. They cannot be induced at will, but 

 must be waited for.^ 



We have thus to decide between : 



1. a selection of extreme variants. 



2. a selection of mutants. 



The question for Darwin was. which of these two 

 has played the greater part in the origin of species ?- 



this deficienc3^ See Vilmorin-Andrieux, Les plantes potageres, 1883, 

 pp. 221-222. 



^ Origin, loc. cit., p. 62. 



^ The selection of extreme variants in nature forms the so-called 

 local races and plays an important part not only in acclimatization 

 but especially in many cases of adaptation to new environmental con- 

 ditions. See III, § 4. 



