MEANS AND METHODS OF EVOLUTIONARY CHANGE 353 



tion). Hence the population tends to increase at a more rapid rate than 

 does the food supply available to that population. Whatever the truth of 

 this much debated generalization, there is no doubt that limited food sup- 

 ply is an important factor in preventing unlimited increase in numbers of 

 individuals in a given species. 



Predatory animals constitute another check on population size. If the 

 lions were exterminated on the plains of Africa the zebra population 

 would doubtless increase rapidly for a time, until a new limit imposed by 

 available food, and perhaps disease, was reached. 



Disease is another limiting factor; epidemics occur among animals, par- 

 ticularly if they become overcrowded. 



Space restrictions form another check on unlimited increase. Not only 

 does overcrowding favor disease and starvation, but a certain amount of 

 "elbow room" is required if animals are to live and reproduce normally. 

 In many species the home or nest is surrounded by a certain area of home 

 territory over which the occupant of the nest dominates and in which all 

 other members of the same species and sex are treated as intruders. The 

 density of population which a given region can support is determined in 

 part, not by the amount of "standing room" available, but by the number 

 of these home territories which can be provided without undue disturb- 

 ance of the normal living and reproductive habits of the species in ques- 

 tion. 



Under the term inanimate environment we may include such limiting 

 factors as climate, seasonal changes, and catastrophes of various kinds. 

 Drought and severe heat in summer and severe cold in winter are exam- 

 ples. It has been estimated that during a severe winter half the wild horses 

 inhabiting the southeastern region of the state of Washington perish. This 

 example may suggest the questions: Is it purely a matter of chance which 

 ones perish and which ones survive? What determines which of these 

 horses shall live, which die? And we are led to the next element in the 

 process of natural selection. 



"The Struggle for Existence" 



We have seen that in each generation each species attempts to produce 

 many more individuals than can hope to live to maturity under the limit- 

 ing conditions prevailing. The result is a competition among the offspring 

 for food, mates, home territories, and the like, and a striving to survive the 

 aggressions of predatory animals, disease, and the severities of inanimate 

 nature. This process was called by Darwin "the struggle for existence." In 



