EVOLUTION 313 



of books of which the first, "Origin of Species," appeared in 

 1859, over 20 years after his return to England. 



Natural Selection. — In 1838 Thomas Malthus published an 

 article on the Law of Population to the effect that since man 

 reproduces in a geometrical ratio, the world would soon be over- 

 populated were it not for checks such as war, famine, disease, 

 etc., which keep down the total numbers. Darwin's theory of 

 natural selection is the doctrine of Malthus applied to the whole 

 animal and vegetable kingdom. The argument of the theory of 

 natural selection may be outlined as follows : 



1. Overproduction. — Animals and plants tend to reproduce 

 more offspring than can possibly grow to maturity. 



2. Struggle for Existence. — Overproduction brings about a 

 struggle for existence, which is largely a struggle for food. 



3. Variation and Heredity. — Offspring of the same parents vary 

 from each other and from the parents; and variations are 

 inherited. 



4. Natural Selection or Survival of the Fittest. — As a result of 

 propositions 2 and 3, the fittest in each generation survive and, 

 in consequence of the gradual accumulation of favorable varia- 

 tions growing out of a slow process of natural selection, a gradual 

 change in species is brought about. 



Overproduction. — That living things reproduce in a geometrical 

 ratio is a self-evident fact that will be admitted by everyone. If 

 an annual plant produced only two seeds — and there is no plant 

 so unproductive as this — in 20 years there would be a million 

 plants. There are numerous cases of a single fish producing 

 millions of eggs in one year. Darwin pointed out that even the 

 slow-breeding elephant, which, beginning at 30 years of age and 

 ending at 90 brings forth 6 young on the average, would at the 

 end of 740 to 750 years produce nearly 19,000,000 elephants. If 

 human breeding were to continue unchecked for say 1,000 years, 

 there would not be standing room on the earth for the progeny. 



Struggle for Existence. — An important limiting factor con- 

 trolling the total population of animals and plants from year to 

 year is food. Doubtless other factors, difficult to evaluate, are 

 also involved. However, since the food supply, under natural 

 conditions, remains practically constant from year to year, a 

 definite limit is set to the numbers of animals and plants that can 



