124 DOCTRINE OF EVOLUTION 



a single rodlike individual taken as a starting-point 

 may be less than one five-thousandth of an inch in 

 length, under natural circumstances it multiplies at a 

 rate which within five days would cause its descendants 

 to fill all the oceans to the depth of one mile. This is a 

 fact, not a conjecture ; the size of one organism is 

 known, and the rate of its natural increase is known, 

 so that it is merely a matter of simple arithmetic to 

 find out what the result would be in a given time. 



Even in the case of those animals that reproduce more 

 slowly, an overcrowding of the earth would follow in 

 a very short time. Darwin wrote that even the slow- 

 breeding human species had doubled in the preced- 

 ing quarter century. An elephant normally lives to 

 the age of one hundred years ; it begins to breed at the 

 age of thirty, and usually produces six young by the 

 time it is ninety. Beginning with a single pair of 

 elephants and assuming that each individual born 

 should live a complete life, only eight hundred years 

 would be requisite to produce nineteen million elephants ; 

 a century or two more and there would be no standing 

 room for the latest generation of elephants. It is only 

 too obvious that such a result is not realized in nature, 

 but it is on account of other natural checks, and not 

 because the natural rate of reproductive increase is 

 anything but excessive. 



The third element of the process of natural selection 

 is the struggle for existence which is to a large extent 

 the direct consequence of over-multiplication. Be- 

 cause nature brings more individuals into existence 



