NATURAL SELECTION 485 



come into existence during the countless ages for which Hfe has 

 existed on the earth. 



It is, to begin with, sufficiently obvious that the available 

 hving space on the earth is, after all, restricted, and there must 

 therefore be a limit to the number of plants and animals that 

 can exist at the same time. We know, however, that all ororan- 

 isms tend to increase in a more or less rapid manner, yet, in a 

 given locality, the numbers of individuals belonging to a par- 

 ticular species remain fairly steady. There must, therefore, be 

 various checks preventing indefinite increase — a constant fight for 

 hfe, a Struggle for Existence. Every plant and every animal is 

 engaged in a keen competition with other forms of life, and 

 has also to battle with the constantly-changing physical condi- 

 tions which collectively constitute climate. Individuals that for 

 any reason surpass others in this constant warfare with their 

 surroundings are, so to speak, selected by Nature to carry on 

 their race, while their less fortunate fellows go to the wall. We 

 have, in short, the active principle of Natural Selection or Survival 

 of the Fittest. 



We have further to consider how and why it is that given 

 individuals are thus favoured in the universal struggle for ex- 

 istence. The classes of facts which give us some insight into 

 this matter may be conveniently arranged in the following tabular 

 statement, followed by a brief discussion of the principles involved. 



PROVED FACTS NECESSARY CONSEQUENCES 



Limited Surface cf Globe and Rapid ") „ , . „ . 



T ■ ^T i_ btrupgle lor Lxistence 



Increase m JN umbers j '^^ 



f Natural Selection or Survival of the 

 Struggle for Existence and Variation • F'ttest 



Natural Selection and Heredity ... Origin of New Species 



Rapid Increase in Numbers. — Darwin takes the elephant as 

 an example of an animal of which the numbers increase with 

 minimum rapidity, a family of six within the space of 60 years 

 being the average, while individuals live for about a century. 

 Supposing all the offspring to survive for the full tenure of 

 existence, this gives a total of about 19,000,000 elephants de- 

 scended from a single pair after the lapse of from 740 to 750 

 years. 



As an example of a species which increases with great rapidity 



vol. IV. 126 



