530 THE EVOLUTION OF LIFE. 



a particular incident force previously unexperienced. Unless 

 the change in the environment is so violent as to be univer 

 sally fatal to the species,, it must affect more or less differently 

 the slightly-different moving equilibria which the members of 

 the species present. Inevitably some will be more stable 

 than others when exposed to this new or altered factor. 

 That is to say, those individuals whose functions are most out 

 of equilibrium with the modified aggregate of external forces, 

 will be those to die; and those will survive whose functions 

 happen to be most nearly in equilibrium with the modified 

 aggregate of external forces. 



But this survival of the fittest * implies multiplication of 

 the fittest. Out of the fittest thus multiplied there will, as 

 before, be an overthrowing of the moving equilibrium wher 

 ever it presents the least opposing force to the new incident 

 force. And by the continual destruction of the individuals 

 least capable of maintaining their equilibria in presence of 

 this new incident force, there must eventually be reached an 

 altered type completely in equilibrium with the altered con 

 ditions. 



1G5. This survival of the fittest, which I have here 

 sought to express in mechanical terms, is that which Mr. 

 Darwin has called &quot; natural selection, or the preservation of 



* It will be seen that the argument naturally leads up to this expression 

 Survival of the Fittest which was here used for the first time. Two years 

 later (July, 1866) Mr. A. R. Wallace wrote to Mr. Darwin contending that it 

 should be substituted for the expression Natural Selection.&quot; Mr. Darwin 

 demurred to this proposal. Among reasons for retaining his own expression 

 he said that I had myself, in many cases, preferred it &quot; continually using the 

 words Natural Selection.&quot; (Life and Letters, &c., vol. Ill, pp. 45-6.) Mr. 

 Darwin was quite right in his statement, but not right in the motive he 

 ascribed to me. My reason for frequently using the phrase &quot; Natural Selec 

 tion,&quot; after the date at which the phrase &quot; Survival of the Fittest &quot; was first 

 used above, was that disuse of Mr. Darwin s phrase would have seemed like 

 an endeavour to keep out of sight my own indebtedness to him, and the 

 indebtedness of the world at large. The implied feeling has led me ever since 

 to use the expressions Natural Selection and Survival of the Fittest with some 

 thing like equal frequency. 



