590 



INTERRELATIONSHIPS OF ORGANISMS 



bond by which man is tied indirectly but effectually to temperature, 

 light, and moisture. 



Medicine is another field of applied biology that must deal with man 

 as an organism and with man's persistent ecological ties. Nearly all 

 disease which is not mere malfunctioning of the machine is some form of 

 host-parasite relationship, often complicated by other organisms that 

 act as carriers, alternate hosts, or "reservoirs" of infection. Malaria, 

 yellow fever, and typhus fever, for example, illustrate intricate inter- 

 specific dependencies that involve not only man as host and the parasite 

 but the intervening roles of such "carriers" as mosquitoes, lice, and 

 fleas, and of still other organisms that may be effectively concerned in 



8300 B.C 



1700 1950 

 AD. A.D. 



Fig. 34.1. The increase in world population from prehistoric times to the present. {After 

 Warren S. Thompson, courtesy Scientific American.') 



the relationship. Medicine, engineering, and sanitation have done much 

 to reduce the toll taken by these and other diseases and have indeed 

 made possible the huge population densities that are an essential condi- 

 tion of modern civilization; but few if any diseases have been eliminated. 

 They are merely held in check or relegated to the more backward regions 

 so long as civilization can empirically or rationally interrupt the ecological 

 chains upon which they depend. 



We have seen that man's cultural, intraspecific relations, because they 

 involve phenomena and activities that are but primitively if at all ex- 

 hibited by other organisms, are the special province of the social sciences 

 and are largely outside of the biologist's professional concern. Even here, 

 however, the concepts of biotic potential and environmental resistance 

 appear to hold, but with the decreasing pressure of the physical and 

 interspecific environment replaced by an ever-increasing intraspecific 

 competition. 



Man cannot afford to ignore his ecological relationships, for he has 

 become an exceedingly powerful ecological agent. His tremendous in- 

 crease in numbers and his cultural inventions (particularly his learning 

 how to exploit the energy accumulated as coal and oil through many 



