212 CAUSES OF VARIATION 



constitution. For example, it is significant that in this series 

 both the solubility in water and the acid strength diminish as 

 the proportion of carbon increases. 



In addition to the properties of non-living units the theory of 

 physiological units as the basis for specific characters in living 

 matter requires one distinctive and additional quality, namely 

 life, with its attendant phenomena, the power of nutrition 

 and growth. But what are nutrition and growth ? Considered 

 in general terms, nutrition is simply the power of one chemical 

 compound (the living) to enter into the composition of another 

 (the food) and break it up and readjust its elements on a basis 

 like its own, leaving the residues to take care of themselves. 



This readjustment of the non-living food to the composition 

 of the living plant or animal we call nutrition, and it means 

 essentially an increase of the living at the expense of the non- 

 living ; in other words, growth through the numerical increase 

 of living units. 



There is often readjustment in the non-living world when two 

 compounds are brought together, the weaker giving way to 

 the stronger affinity, but there is no such wholesale ''carry- 

 ing over" of matter from one to another as in the phenomena 

 we call nutrition and growth. This is a true invasion of the 

 non-living by the living world, transferring matter almost indef- 

 initely, unto itself, not only preserving its own identity in the 

 meantime but impressing it upon the appropriated materials 

 as well. 



These physiological or vital units are therefore conceived to 

 be the smallest living units, like molecules in non-living matter, 

 except that they are far more complex in constitution and are 

 endowed with the power of self-multiplication through nutrition. 

 This requires growth and division after the manner which has 

 been noted in chromatin granules, except on a scale infinitely 

 more minute. 



This conception of the action of living units has, its similitude 

 in the non-living. Crystallization is a growth, in the sense of 

 increase of size, but it is not attended by transformations equal 

 to those in living matter. Furthermore, crystallization is growth 

 without differentiation, except as to geometric form. Either 



