Growth and Form 105 



(i.e., would be like equation 5) and would yield the first part of the 

 S-shaped curve. As growth proceeded and N became larger, bN would 

 become larger and begin to detract appreciably from a. Thus the rate of 

 growth would progressively decrease. When N became so large that bN 

 equalled a, the term (a — bN) would be zero, the rate of growth would 

 be zero, and growth would cease. The constant, b, could be used to de- 

 scribe the decrease in the food supply or accumulation of toxic wastes that 

 inhibit growth. In the case of the growth of organisms that serve as prey 

 for other organisms, b could be used to describe the way in which the pred- 

 ator cuts down the growth of the prey (by kilUng off potential parents). 

 In this case, b would not be a constant but would depend on the number 

 of predatory organisms. Finally, b might be used to account for epidemics 

 that kill substantial numbers of organisms and would be treated in much 

 the same way as if a predator were feeding upon the population. If we 

 were studying the growth of an organ, say the number of liver cells in a 

 human, b might also be used to describe the action of hormones from 

 other parts of the body upon the liver cells. 



