STUDIES, SCIENTIFIC AND SOCIAL 



such as Lord Salisbury, alike suffer in this respect, it is 

 needful to again state obvious facts which may serve to 

 drive home the overwhelming importance of this factor in 

 evolution. 



Let us suppose an animal which lives ten years and 

 produces ten young (five pairs) each year, a moderate 

 allowance even for many mammals and birds. A little 

 simple arithmetic will show that if none died for five 

 years there would be 6,480 pairs in place of the one pair, 

 or, 6,480 millions in place of one million, as the case 

 might be. But it is evident that such an average rate of 

 increase for all animals could not go on for even one or 

 two years, as no country could supply them with food. 

 We will suppose, then, that only one pair, instead of five, 

 survive each year to breed the next year ; but if this goes 

 on for the ten years of the life of the first pair we shall 

 still have 512 pairs instead of each pair, a number which 

 is equally impossible. Let us, then, suppose that only 

 one-fiftieth part of those born survive, that is, that only 

 one individual lives to breed out of five successive broods 

 of ten each ; even then, at the end of ten years, we shall 

 have a population two and a half times as great as at 

 first, or, more exactly, if we began Avith a million in- 

 dividuals, then in ten years we should have 2,593,743. 

 This is probably something like what happens. Forty- 

 nine fiftieths of those born never live to breed, yet the 

 population increases steadily so long as conditions are 

 moderately favourable, the surplus being got rid of at un- 

 certain intervals by recurrent unfavourable conditions, so 

 as to keep the number of individuals on the average 

 about stationary. Looking at it in another way, we find 

 that, beginning with 100 individuals whose offsprings each 

 year amount to 500, of which only 10 survive to breed, 

 then during ten years about 8,000 will have been born, 

 making with the original hundred, 8,100, out of which 

 only the 100 fittest, or nearly the fittest, will survive, to 

 be again weeded out every successive ten years, or there- 

 abouts. Without making some numerical estimate of 

 this kind it is impossible to realise the severity of the 

 struggle continually going on in nature and the resultant 



