THEORY OF EVOLUTION 507 



his studies, many of which were done along the east and west shores 

 of South America while he was naturalist of a British Naval ex- 

 pedition on the ship Beagle, Darwin formulated a clear-cut and 

 definite argument for evolution on the basis of natural selection. Be- 

 ginning with Malthus' law of population, published in 1838, which 

 stated that since man reproduces in a geometric ratio, the earth would 

 be overpopulated in a few generations except for such checks as the 

 arithmetic ratio of increase in food production, disease, war, flood, 

 earthquake, fire and other natural catastrophes reducing population, 

 Darwin formulated the theory of natural selection. This theory in- 

 cludes among other things the application of Malthus' law to all living 

 organisms. The four basic points on which this theory is developed 

 may be named in order as follows: (1) overproduction, (2) struggle 

 for existence, (3) variation and heredity, and (4) survival of the 

 fittest (natural selection). 



Overproduction is in operation in all thriving normal species. A 

 single codfish will produce several million eggs in one season. If 

 every codfish egg were to be fertilized, to reach maturity, and to re- 

 produce with no loss from one generation to the next, it would not be 

 more than a dozen years until the entire face of the earth would be 

 covered with codfish and all other animals would be crowded out of 

 existence completely. Even a form like the elephant, which lives to 

 be ninety or a hundred years of age and averages only six progeny, 

 could soon occupy all of the standing room on the face of the earth. 

 Beginning with one pair of elephants and providing every individual 

 lived and reproduced even at the slow average rate mentioned above, 

 19,000,000 individuals would be produced in 750 years. If every 

 elephant alive today were to enter into a program like that, both food 

 and space would become quite scarce before "many generations. How- 

 ever, this doesn't happen on a large scale. All plants and animals 

 tend to produce more offspring thafi can ever reach maturity and 

 reproduce. 



The struggle for existe^ice is ever present because there are more 

 individuals produced than the habitat will support. The two most 

 fundamental needs for which organisms struggle are (1) food and (2) 

 opportunity to reproduce and rear young. Of these two, the struggle 

 for food is very immediate and the food supply is an important 

 limiting factor on population from season to season. Since the food 

 supply, on the average, remains quite constant, it is evident that 



