ECOLOGY AND EVOLUTION 187 



What has been said about fluctuations in numbers shows that 

 such is not always the case. Many animals periodically 

 undergo rapid increase with practically no checks at all. In 

 fact, the struggle for existence sometimes tends to disappear 

 almost entirely. During the expansion in numbers from a 

 minimum, almost every animal survives, or at any rate a very 

 high proportion of them do so, and an immeasurably larger 

 number survives than when the population remains constant. 

 If therefore a heritable variation were to occur in the small 

 nucleus of animals left at a minimum of numbers, it would 

 spread very quickly and automatically, so that a very large 

 proportion of numbers of individuals would possess it when 

 the species had regained its normal numbers. In this way 

 it would be possible for non-adaptive (indifferent) characters 

 to spread in the population, and we should have a partial 

 explanation of the puzzling facts about closely allied species, 

 and of the existence of so many apparently non-adaptive 

 characters in animals.^^ 



II. There are many objections to this hypothesis, which is 

 chiefly mentioned here, not merely because it affords a possible 

 solution of the problem of the origin of species, but because 

 it illustrates the fact that ecological studies upon animal 

 numbers from a dynamic standpoint are a necessary basis for 

 evolution theories. Another important result of the periodic 

 fluctuations which occur in the numbers of animals is that the 

 nature and degree of severity of natural selection are periodic 

 and constantly varying. For instance, at a minimum of 

 numbers, rabbits will undergo selection for resistance to bad 

 climate or ability of males to find females, while at times of 

 maximum there will be different types of selection, e.g. for 

 resistance to disease, and ability of males to secure females 

 in competition with other males. All these points about 

 adaptation, numbers, and selection, prove that ecological work 

 has a very important contribution to make to the study of the 

 evolution problem. 



