Variation and Adaptation. 149 



— according to the old acceptance of the term nutrition. 

 Their effect can be intensified, in the case of partial as 

 well as in that of individual variability, in the course of 

 a few generations whether these be sexually or vegeta- 

 tively produced. 



§ i8. VARIATION AND ADAPTATION. 



It has often been maintained that groups of individ- 

 uals which vary are better adapted to a changing environ- 

 ment than groups of individuals which are all alike. 



Variability must not, however, in my opinion be re- 

 garded solely as an adaptation. But the fact that the 

 amplitude of variation, the Abdnderungsspielrauni as 

 Ammon has happily termed it,-^ is very different in dif- 

 ferent organs and characters and also in different species 

 of animals and plants suggests that there must be a 

 definite cause for it in each particular case. 



The form of a Ouetelet^s curve is determmed by 

 two factors; the magnitude of the mean value and the 

 amplitude of variation. We are accustomed to regard 

 as a measure of the latter that part on the base line 

 which includes half of the individuals between the mean 

 and the furthest scale character. The mean (M) and the 

 amplitude (O) are independent values. But they are 

 both specific characters at least in every case in which 

 they deviate from the normal. And it is not necessary to 

 say here that specific characters are at least in very many 

 instances adaptive characters. 



I shall now examine two very important phenometia 

 from this point of view. First, the frequently enormous 



^ Otto Ammon, Dcr Ah'dnderungsspielraum. Naturwissensch. 

 Wochenschrift, 1896, Nos. 12-14. 



