82 EVOLUTION 



we may say that the plant-cell, by shutting 

 itself up in a wall of cellulose, instead of 

 fully oxidizing this substance, and perhaps 

 also by less efficient elimination of nitro- 

 genous waste, doomed itself to fixity and to 

 sleep. Yet something of the animal impulse 

 of the ancestral Protists lingers in the plant, 

 and something of the vegetative tendency of 

 the ancestral Protist lingers in the animal. 



We have dwelt for a little on this elementary 

 question of the distinctions between plants 

 and animals, because it is the fundamental 

 illustration of a bifurcation that has recurred 

 many times in the evolution of living creatures. 

 Living implies two great processes of re- 

 pairing and wasting, of building up and 

 breaking down, of construction and dis- 

 ruption more technically, of anabolism and 

 katabolism. Given a typical plant and 

 animal of equal weight, both living normally, 

 we might safely say that the animal lives 

 much more nearly up to its income than the 

 plant does. If we express the vital ratio of 



A 



anabolism to katabolism as =. for the plant 



Iv 



and - for the animal, we may safely say 

 that ~ is always much greater than -. In 



JV K 



