100 Growth and Form 



a bacterial culture, or very dramatic, as when populations of Alaskan lem- 

 mings exhaust their food supply and in the course of frantic migration 

 commit mass suicide by leaping into the ocean. 



2. The environment becomes too toxic for further growtli. Organ- 

 isms, as they grow, pour out metabolic waste products and thus pollute 

 the environment. As long as the number of organisms is small, the con- 

 centration of wastes will be low and will not poison them. When the 

 number is large, the rate of waste production will rise and the concentra- 

 tion in the environment will increase to a toxic level and lower the vitality 

 of the organisms to the extent that they can no longer grow or perhaps 

 even survive. 



We should mention a special case under this second category that 

 is particularly pertinent for animal populations, namely the stoppage of 

 growth not by toxic materials but by epidemics caused by parasites. 

 Epidemics are like nuclear chain reactions. That is, the host population 

 must rise above a critical density before the epidemic can take place. ( Can 

 you construct an explanation of this?) Thus, when an animal population 

 does grow to great numbers, an epidemic can strike and reduce the pop- 

 ulation drastically. Then another cycle of growth will begin. Biologists 

 have charted such growth cycles for many wild animal populations. 



Growth of the Multicellular Organism 



A population of microorganisms is a collection of discrete cells that 

 are largely independent of each other for their existence. The higher ani- 

 mal is itself simply a collection of discrete cells that is organized into a 

 compact whole, but the cells are closely dependent upon each other for 

 their continued existence. To the degree that the animal is simply a collec- 

 tion of discrete cells, it grows according to the same rules as does a micro- 

 bial culture. But the interdependence of cells in the animal body brings 

 complications: 



1. All parts of the animal do not grow at the same rate. Some tissues 

 do not grow at all once the embryo is formed. Others grow very slowly. 

 Still others grow very rapidly. The total organism, however, which is the 

 resultant of all the individual cells and tissue, does grow precisely as do 

 populations of microorganisms, i.e., in an S-shaped curve. 



2. All parts of the organism do not stop growing simultaneously. In 

 fact, some tissues continue to grow during the entire lifetime of the ani- 

 mal. However, the total organismic mass does remain relatively constant 

 once maturity is reached ( in the absence of pathological changes ) . 



