352 INTRODUCTION TO EVOLUTION 



It was Darwin's thought that nature selects animals and plants in much 

 the same manner, preserving those individuals which have characteristics 

 best fitting them for life in the particular environment in which they find 

 themselves and eliminating individuals less adequately equipped. If such 

 natural selection does indeed occur, what is the driving force back of it 

 and by what means is it accomplished? 



Tendency to Rapid Increase in Numbers 



The driving force, according to Darwin, is provided by the tendency of 

 all living things to increase their numbers rapidly. A few examples will 

 typify prevailing situations in most animals. Fishes are noted for the laying 

 of large numbers of eggs. A 25-pound carp in an Iowa lake was found to 

 contain 1,700,000 eggs; the similar prodigality of the salmon in egg produc- 

 tion is common knowledge. One female toad may lay as many as 12,000 

 eggs. It has been calculated that one pair of houseflies breeding in April 

 would have by August, if all eggs hatched and all resulting individuals 

 lived to reproduce in their turn, 191,010,000,000,000,000,000 descendants. 

 Turning to animals which breed more slowly and have longer intervals 

 between generations, we may quote Darwin's statement concerning ele- 

 phants: "The elephant is reckoned the slowest breeder of all known ani- 

 mals, and I have taken some pains to estimate its probable minimum rate 

 of natural increase; it will be safest to assume that it begins breeding when 

 thirty years old, and goes on breeding till ninety years old, bringing forth 

 six young in the interval, and surviving till one hundred years old; if this 

 be so, after a period of from 740 to 750 years there would be nearly nine- 

 teen million elephants alive descended from the first pair." Other examples 

 might be quoted almost endlessly. 



Limiting Factors 



Why, in actual fact, do we not find our lakes choked solidly with fish, 

 our fields carpeted with toads, the earth overrun with elephants, and so 

 on? Because there are for each species certain checks or limiting factors 

 opposing such increase in numbers. 



One of the most important of these checks is limited food supply. Dar- 

 win himself was greatly influenced in his thinking by the essay of Malthus 

 on population. It was the thesis of Malthus that population tends to in- 

 crease in geometric ratio (e.g., by successive multiplication) while the 

 food supply tends to increase in arithmetic ratio (e.g., by successive addi- 



