444 THE EVOLUTION OF LIFE. 



turbations and deviations, which, when compounded with one 

 another from generation to generation, work innumerable 

 slight modifications in the moving equilibria and correlative 

 structures throughout the species. 



Now if the individuals of a species are thus necessarily 

 made unlike, in countless ways and degrees if the compli- 

 cated sets of rhythms which we call their functions, though 

 similar in their general characters, are dissimilar in their 

 details if in one individual the amount of action in a par- 

 ticular direction is greater than in any other individual, or if 

 here a peculiar combination gives a resulting force which is 

 not found elsewhere ; then, among all the individuals, some 

 will be less liable than others to have their equilibria over- 

 thrown by a particular incident force, previously unexperi- 

 enced. Unless the change in the environment is of so vio- 

 lent a kind as to be universally fatal to the species, it must 

 affect more or less differently the slightly different moving 

 equilibria which the members of the species present. It 

 cannot but happen that some will be more stable than others, 

 when exposed to this new or altered factor. That is to say, 

 it cannot but happen that those individuals whose functions 

 are most out of equilibrium with the modified aggregate of 

 external forces, will be those to die ; and that those will sur- 

 vive 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 

 that are the least capable of maintaining their equilibria in 

 presence of this new incident force, there must eventually be 

 arrived at an altered type completely in equilibritim jrith the 

 altered conditions. 



165. This survival of the fittest, which I have 



