Summary. 575 



fore, caused by these two factors. The amount of hte devi- 

 ation of any given character from its mean is determined 

 partly by selection, i. e., by the characters of its parents and 

 grandparents and partly by nutrition, i. e., by the ojjcration 

 of external influences on the individual itself. But the 

 characters of the ancestors were also determined by the 

 conditions of life; so we arrive at the conclusion that 

 the phenomena of variability in the strict sense of the 

 term, that is, the individual deviations from the mean 

 of the species are solely caused by external conditions. 

 Only it must be remembered that nutritional influences 

 may be cumulative over several generations, inasmuch 

 as only the best individuals will bear the best seed. 



Fluctuating variability therefore falls within the 

 province of the physiology of nutrition. The external 

 causes of mutation are, on the other hand, as yet wholly 

 unknown. 



