To the parasitic and predatory organisms the situation is an exceptional 

 period of prosperity and expansion. For the victims, however, it is an epi- 

 demic, a visitation of misfortune. On the other hand, a period of prosperity 

 and expansion is Hkely to be followed by a period of privation, as, for example, 

 with the ladybirds who have all but exterminated the scale insects in a region. 

 Moreover, long before most of the prosperous and abundant Httle beetles have 

 a chance to suffer from famine, they will have furnished a feast for various 

 birds and their babies. 



An epidemic usually comes to an end abruptly because the successful 

 species has destroyed its own food supply. In the case of insect or fungus pests, 

 an epidemic — that is, an unusual crowding — invites another species to take 

 advantage of the unusual abundance of food. In the long run the victor be- 

 comes the spoils. 



How Has Man Disturbed the Balance of Nature? 



Man's Intrusion^ Long before the dawn of history man had domesti- 

 cated the dog and species of ox, sheep and goats. He was able to maintain a 

 steady food supply. The family was enabled to enlarge, and to stay in one 

 place for a relatively long period. Herdsmen did, of course, have to move 

 when rains failed or when their cattle ate up all the grass in the neighbor- 

 hood. But the nomads were more orderly in their rovings than hunters. 

 Living generally became better organized. 



From being a hunter to being a herdsman man took a step forward. From 

 being a herdsman to settling down as a soil- tiller, he took another step for- 

 ward. The gains may be measured by the fact that population grew. The 

 domesticated plants and animals multiplied in numbers. But man's success 

 threatened to upset the natural balance. Increasing the population of men 

 and of domesticated species furnished their enemies and parasites exceptional 

 opportunities. Flies and liver-flukes increased rapidly. Man has invited to 

 his farms all kinds of vermin, insects, fungi and worms that had previously 

 Uved on the sparse vegetation or animals of the natural life-community. 



These changes in man's mode of life meant more intensive hunting of birds 

 and game and fish. They meant changing the composition of the streams into 

 which he threw his refuse. In proportion as man has thrived and grown in 

 numbers, he has made increasing demands upon the earth and has exerted 

 increasing pressure upon other species. Concentrating population — human, 

 vegetable, animal — brought about the destruction of some species and the 

 increase of others. 



Mining Wood Man could find an opportunity to live only in a region 

 at or near the climax of its development (see page 568). The forest has been 



iSee Nos. 1 and 2, p. 598. 

 582 



