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 kirid 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 : 
“Tf under changing conditions of life organic beings 
