Theory of Natural Selection 
The theory of natural selection is so generally 
understood, that to set it forth in detail in this 
place would be quite superfluous. 
Darwin, it will be remembered, based his 
great hypothesis on the following observed 
facts :— 
1. No two individuals of a species are exactly 
alike. This is sometimes called the law of 
variation. 
2. All creatures tend in a general way to 
resemble their parents in appearance more 
closely than they resemble individuals not re- 
lated to them. This may be termed the law of 
heredity. 
3. Each pair of organisms produces in the 
course of a lifetime, on an average, many more 
than two young ones. 
4. On an average the total number of each 
species remains stationary. 
From (3) and (4) follows the doctrine of 
Malthus, namely, that many more individuals 
are born than can reach maturity. 
Darwin applied this doctrine to the whole of 
the animal and the vegetable kingdoms. 
In his introduction to Zhe Origin of Spectes 
he writes :—‘‘ As many more individuals of each 
species are born than can possibly survive; and 
as, consequently, there is a frequently recurring 
struggle for existence, it follows that any being, 
if it vary, however slightly, in any manner pro- 
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