THE STRUGGLE FOR EXISTENCE 21 



many more individuals are born than can possibly sur- 

 vive. The result is that those born with certain weak- 

 nesses or under unfavorable conditions are the ones which 

 are most likely to die, while those possessing greater 

 strength or born under favorable conditions are the ones 

 most likely to live. Hence it is that there tends to be 

 a survival of the fit. Nature, so to say, selects the best 

 to survive. 



It is a self-evident fact that the amount of space upon 

 the earth is limited. At first thought it is not so evident 

 that living things tend to multiply in geometrical pro- 

 gression. But the truth of this principle is easily demon- 

 strated. Romanes tells us that if the progeny of a single 

 pair of elephants, which are the slowest breeding of ani- 

 mals, were allowed to reach maturity and propagate, in 

 750 years there would be living 19,000,000 descendants. 1 

 Professor Metcalf has computed the following table based 

 upon the rate of increase of the common robin. Sup- 

 posing that the yearly offspring of each pair of robins is 

 four on the average, which is below the usual number, 

 then a single pair of birds would have four young in the 

 first generation. 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. 

 In twenty years the descendants of the original pair 

 would number over twenty billion ! 2 



This should make it clear that the earth could not sup- 

 port the progeny of even a single species if the natural 

 increase were allowed to go unchecked. 



But in the case of the robins, more birds die each year 

 than live because we find that the number remains con- 



1 Romanes, G. J. Darwin and After Darwin, I The Darwinian Theory, 

 1901, p. 261. 



2 Metcalf, op. cit., p. 14. 



