from season to season. Plants grow but are constantly destroyed by other 

 plants and by animals. Baby suiifish increase in size, while the insect larvae 

 on which they feed diminish in numbers. They themselves diminish in num- 

 bers, while a perch grows at their expense. 



At the end of a good growing season insects and worms and birds and ro- 

 dents, as well as plants, will be more numerous than after a poor growing 

 season. That in turn will mean a prosperous year for hawks and foxes and other 

 carnivorous animals. Later on various fungi, worms, beetles and bacteria will 

 be exceptionally numerous. A species expands to the limit of exceptional 

 abundance only to furnish a stroke of luck for those who depend upon it. 

 There is no endless up and up; life is a succession of ups and downs. Even 

 in the steady growth of an old tree we can find indications that its "fortune" 

 has fluctuated with changes in the amount of sunshine (see illustration 

 opposite). These records are so consistent that it has been possible to ascertain 

 the dates of timbers in ancient structures through them. And it has been 

 suggested that human affairs might be profitably studied in terms of the 

 changing abundance of plant and animal life in the past. 



Food and Elbowroom Experiments with flour-beetles and other in- 

 sects show that in a given area the number of individuals never increases past 

 a certain point regardless of the amount of food. A colony of bacteria in a 

 food medium will grow only so far and then stop, long before exhausting the 

 food. Apparently there is a point beyond which more and more food does not 

 mean more and more growth — for a particular individual or for a colony of 

 individuals or for a species. 



In a given field a thousand seeds of corn or of tomato will start more plants 

 than five hundred seeds. But five hundred may produce a greater number of 

 mature individuals and a greater yield. For spacing and air are quite as es- 

 sential as root-hold. With human beings food is a first condition for growing 

 and multiplying, and elbowroom is a close second. And yet the race appears 

 to have multiplied more rapidly where the density of population is already 

 highest. In slums of industrial cities and in parts of India, for example, the 

 birth rate is higher than in other parts of the community. Yet in many such 

 places the death rate exceeds the birth rate. People continue to live there 

 only because new individuals and families are constantly being pushed in from 

 outside. Such crowding of one species offers very favorable opportunities for 

 other parasitic or predatory species. 



Epidemics People in past ages looked upon epidemics of disease or of 

 pests exactly as many of us today look upon an unexpected hurricane or earth- 

 quake. They just happen. In the quaint language of insurance company 

 lawyers, they are described as "acts of God" — without necessarily implying 

 either any theory as to how things come to happen or any theory of religion. 



Today we do have definite theories about how epidemics come about, and 



580 



