374 PROGRESSIVE DEVELOPMENT 



made after reading Mai thus' Essay on Population and from mate- 

 rial gathered during his trip around the world on a ship commis- 

 sioned for. scientific exploration. It consists of the following 

 principles. (1) Overproduction. More individuals are produced 

 than can possibly reach maturity and reproduce themselves. A 

 plant commonly called the shepherd's purse may produce upon a 

 single stalk as many as 64,000 seeds, and a tobacco plant may pro- 

 duce 360,000 seeds annually. A single fern plant produces about 

 fifty million spores a year. If all these spores matured, the United 

 States would be covered with ferns in two years. A single salmon 

 may lay 2,000,000 eggs, a female codfish 9,000,000 eggs, and a Vir- 

 ginia oyster not less than 15,000,000 eggs. The sea would be a 

 mass of writhing, struggling fish in three years if all survived. This 

 overproduction leads to (2) a struggle for existence. Whenever 

 there are abundant offspring, there must be ample food and room 

 for life and development. Crowding makes the existence of indi- 

 viduals a real contest or fight. A struggle ensues for obtaining 

 food, for finding a mate, and for producing more young. All can- 

 not survive. Those that do are better adapted to meet the 

 conditions of their environment than the rest. It is a well-known 

 fact that all the individuals of a species are not the same. Some 

 are swift and some skillful, which help them to survive. These 

 slight differences are known as (3) variations. These variations 

 play a most important part in the struggle for existence. Each 

 one that is swift of foot, strong, skillful, or protectively colored 

 possesses a variation that fits him to his environment better than 

 those that do not possess such a variation. Variations make 

 animals unequal in the contest of life. There is a struggle for 

 existence : some are killed off ; others, the fittest animals or 

 plants, survive. Thus overproduction intensifies the struggle for 

 existence ; this struggle results in (4) the survival of the fittest. 

 Darwin's theory of selective survival is often called "natural 

 selection." (5) Variations are usually inherited. Those slight 



