52 ANIMAL INDIVIDUALITY [CH. 



because these substances its food may be varied 

 to a considerable extent and the end-result, its 

 protoplasm, yet be the same. A crystal on the other 

 hand, cannot build up the complex from the simple 

 it can only add ready-formed molecules to its 

 substance, and can only use them if they are presented 

 to it in one particular way, in the condition of a 

 saturated solution. 



An organism is more definite because its size is 

 defined as well as its form. A crystal will continue 

 to grow without limit if only the appropriate mother- 

 liquor in which it hangs is kept saturated : its form is 

 definite, its size indefinite. It is a fact of common 

 observation, however, that each organism has a 

 typical size not invariable, but fixed within certain 

 not very wide limits. This again is due to the 

 differences in the modes of assimilation of crystal 

 and living thing. In the crystal, growth takes 

 place entirely at the surface. Its assimilation is 

 purely physical: it assimilates to its own physical 

 state molecules of the same chemical composition but 

 in a physical state different from its own. Capturing 

 molecules from their state of solution, it builds them 

 up on its solid self in such wise that they fit on to the 

 pattern of the already existing structure. 



With protoplasm, however, assimilation is chemical 

 as well as physical, and growth takes place by intus- 

 susception, not by accretion. That is to say, it works 



