NATURAL SELECTION II 



The never ending, ever stressful struggle for life is a 

 direct and necessary result of two facts: first, that the 

 amount of food and the space to be occupied on the earth 

 by animals and plants are limited ; and, second, that the 

 processes of reproduction, if unhindered by any adverse 

 circumstances, would give a geometrical ratio of increase 

 of plants and animals. Let us look a moment at the sec- 

 ond of these two propositions. The first, that the earth 

 is capable of supporting only a limited number of living 

 things, is, of course, understood without illustration, but the 

 facts of geometrical ratio of increase in animals and plants, 

 unless opposed by unfavorable conditions, are worth illus- 

 trating. Our common American animals and plants give 

 us as good examples as we could wish. 



The common robin raises annually one to three broods, 

 of three to six young in each brood. Say that the yearly 

 offspring of each pair of birds is four on the average, 

 which is surely a low estimate, then a single pair of robins 

 would have in the first generation four young. The second 

 year they would have four more young, and their young 

 of the first year, mating, would have eight young, four for 

 each of the two pairs. If for ten years the original pair 

 and all of their offspring were to live and reproduce at 

 the assumed rate, four young a year for each pair of adults, 

 then at the end of the tenth year there would be over one 

 hundred thousand robins, all descendants of the first pair. 

 (See Table.) 



Adults Young 



One pair of adult robins .... 2 



Fiist year, their young ..... 4 



Second year ....... 6 12 



Third year 18 36 



