RATE OF INCREASE 47 



Let us also consider the case of a minute rapidly 

 breeding animal of a typical kind. My friend Mr. 

 R. C. Punnett has recently been engaged upon an 

 experiment which involved the breeding of rotifers, 

 a kind of animal barely visible to the naked eye. 

 They were bred for sixty-seven generations, and 

 each individual produced on the average thirty eggs. 

 The whole experiment occupied less than a year, yet 

 Mr. Punnett calculated that if he had been able to 

 rear all the animals which, at this rate of breeding, for 

 this number of generations! were theoretically obtain- 

 able, he would have become the possessor of a solid 

 sphere of organic material with a radius greater than 

 the probable limits of the known universe. 



This geometrical rate of increase is common in a 

 greater or less degree to all living organisms. Since 

 the space and food-supply available for the support of 

 any species has no corresponding tendency to in- 

 crease, it follows that a large proportion of the 

 individuals born must perish before they reach the 

 adult state, or at least without producing offspring. 

 Darwin's contention is that there will be a strong 

 tendency for those individuals which show slight 

 modifications in the direction of a better adaptation 

 to their environment to survive at the expense of 

 those of their brethren which do not exhibit similar 

 modifications. This is the principle called natural 

 selection by Darwin, and by Herbert Spencer the 

 survival of the fittest. Let us quote Darwin's own 

 summary of the process g 



' If under changing conditions of life organic beings 



