158 THE LAW OF BIRTHS AND DEATHS 



Doubleday, become unusually fertile, and the next genera- 

 tion will be more numerous rather than less numerous. . . . 



" Thus there will go on an ever increasing rate of multi- 

 plication, and an ever decreasing share of food for each 

 person, until the species disappears. Suppose, on the 

 other hand, the members of a species to be in an unusually 

 plethoric state. Their rate of multiplication, ordinarily 

 sufficient to maintain their numbers, will become in- 

 sufficient to maintain their numbers. In the next genera- 

 tion, therefore, there will be fewer to eat the already 

 abundant food which, becoming relatively still more 

 abundant, will render the fewer members of the species 

 more plethoric and still less fertile than their parents. 

 And the actions and reactions continuing, the species 

 will presently die out from absolute barrenness." l 



This criticism shows a failure to grasp the most essential 

 aspects of the problem. Take the case of wild animals. 

 The food supply is dependent mainly on the seasons. 

 A bad season would lead to a heavy deathrate. In the 

 case of the larger species this would produce a higher 

 degree of fertility ; but the births resulting would in most 

 cases fall during the next season, thus compensating 

 in some measure for the heavy deathrate of the previous 

 year. There would, of course, be no accumulation of 

 food ; but the next season might be a very favourable one. 

 In the case of a very favourable season with a reduced 

 deathrate, this would lead to a reduced birthrate next year. 

 The principle provides for the case of absolute famine. 



It is in the case of the higher animals that the principle 

 will be of most importance. The higher we ascend in 

 the scale of life, the more important the principle becomes, 

 since slow-breeding animals have a much smaller margin 

 between maintaining the species and extinction. In the 

 1 Principles of Biology, Spencer, vol. ii, part vi. 



