GREAT STEPS IN EVOLUTION 81 



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 ele- 

 mentary question of the distinctions between 

 plants and animals, because it is the funda- 

 mental illustration of a bifurcation that has 

 recurred many times in the evolution of 

 living creatures. Living implies two great ^ 

 processes — of repairing and wasting, of build- ; 

 ing up and breaking down, of construction 

 and disruption — more technically, of ana- 

 bolism 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 anabolism to katabolism 



as — for the plant and - for the animal, we 



may safely say that — is always much greater 



ii. 



than — In the plant the numerator is 



always large in proportion to the denomi- 

 nator. In the animal there is a relative 

 preponderance of katabolic processes. Thus 



