HISTORICAL ACCOUNT OF EVOLUTION THEORY 25 



of Nature, however, dominated all other interests and shortly after 

 graduation an opportunity came to join the ship 'Beagle' as naturalist 

 in a voyage of exploration around the world. The five years spent 

 upon this memorable journey, the narrative of which is so admirably 

 set forth in the book, A Naturalist's Voyage around the World, resulted 

 in the accumulat on of the first of Darwin's great series of observations, 

 the final decision to devote his life to zoological research, and the 

 beginning of that illness which made him a life-long invalid. This 

 last factor necessitated a retired life and thus proved of indirect bene- 

 fit, as it enabled him to accomplish the immense amount of work 

 which he did without being impeded by the distractions of a public 

 career." 



SUMMARY OF DARWIN's THEORIES 



Since two subsequent chapters are to be devoted to Darwinism, 

 only an outline of Darwin's theories need be presented in the present 

 historical account. 



Although Darwin was an all-round biologist and gave attention 

 to practically every phase of evolutionary biology, he is known espe- 

 cially for his selection theories'. There are three of these : the theory 

 of artificial selection, the theory of natural selection, and the theory of 

 sexual selection. 



-. a) Artificial selection. — According to Darwin the commonest 

 method of producing, under human culture, new races of animals and 

 plants is that of selection. The breeder selects from among the highly 

 variable individuals of a parent-race those which possess the begin- 

 nings of desired modifications, and he breeds them together, expecting 

 that the offspring will show the desired character, some in a more 

 highly perfected condition, others in a less. The ones that vary 

 favorably are again selected for breeding stock, and the same process 

 is carried on until the desired character has been perfected. 



Although we now know that this is far from being a typical experi- 

 ence among breeders, it appeared to Darwin to be so typical that he 

 transferred the selection idea from the breeder to Nature, making 

 Nature the selecting agency responsible for the production of natural 

 wild species. His argument is as follows: 



b) Natural selection. — The following factors are involved : 

 ^ I. All animals and plants tend to multiply in geometrical ratio. 



2. There is not food or room for a much larger number of animals 

 and plants than now exist. 



