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 theaccumulat on of the first of Darwin’s great series of observations, 
the final decision to devote his life to zodlogical 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: 
1. 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. 
