422 THEORIES OF EVOLUTION 



result is that some are better fitted to their surroundings 

 than others, which results in (/) the survival of the fittest 

 in the struggle for existence. 



All these factors, with one exception, were recognized 

 by several biologists before the time of Darwin. Indeed, 

 Darwin's grandfather, Erasmus Darwin, specifically men- 

 tions them in his writings. The supreme work of the 

 grandson was supplying the evidence for the survival of 

 the fittest, which may be considered the cornerstone of 

 the most generally accepted theories of evolution at the 

 present time. These various factors will now be con- 

 sidered in more detail. 



Overproduction. — " There is no exception to the rule 

 that every species of organic being, animal or plant, 

 naturally increases at so high a rate that if none were 

 destroyed, the earth would soon be covered by the prog- 

 eny of a single pair." If, for example, every human being 

 were to come to maturity and were to breed at the average 

 rate, the numbers of individuals would be doubled every 

 fifty years. At the end of a few thousand years, if 

 these conditions prevailed, there would not be standing 

 room for the descendants. An annual plant producing 

 two seeds only would have 1,048,576 offspring in twenty- 

 one years if each individual attained maturity and repro- 

 duced. The conger eel lays 15,000,000 eggs each year, the 

 codfish 9,000,000, the Virginia oyster not fewer than 

 15,000,000 on the average. If these were to come to 

 maturity and breed, their progeny would fill the sea in 

 a very brief period. This factor, therefore, in the theory 

 of evolution is a generally recognized and firmly estab- 

 lished fact. 



Stationary Numbers of Individuals. — It is also well 

 established that the number of individuals of the ma- 

 jority of species remains fairly constant over long periods. 

 There may be an increase at one time and a decrease at 

 another, but in the long run the average is approximately 

 uniform. This is to be expected when it is remembered 



