.CHAPTER X 



EVOLUTION (VARIATION AND SELECTION) 



§ I, — TJie Need for Definition 



In the last chapter we freely used the words " evolution " 

 and " selection " as if they had current common values. 

 Now this is very far from being the case, and it is 

 accordingly desirable to give to these terms and to other 

 subsidiary terms definite and consistent meanings. It is 

 only within the last few years, however, with the growth 

 of a quantitative theory of evolution, that precise definition 

 of fundamental biological concepts has become possible. 

 To the writers who talk of this result or that being due 

 " to the relative variability of local races," who assert that 

 a peculiarity is " a result of the correlation of two organs," 

 or who attribute this or that change of character to 

 heredity, to reversion or to telegony, we now simply say : 

 What is the numerical value of the variability of which 

 you speak ? Have you a measure of this correlation ? 

 Did you test the magnitude of the inheritance of that 

 character ? What is the nature of inheritance in the case 

 of the character which you attribute to reversion or to 

 telegony ? Till very definite answers are forthcoming to 

 these questions, we are not in the present state of our 

 knowledge bound to pay much attention to those who are 

 over ready to " explain " not only organic but social 

 changes by a vague use of undefined biological terms.^ 



^ More than one sociological work has in the last few years obtained con- 

 siderable reputation by applying the Darwinian theory without the least 

 quantitative investigation to human societies. See the remarks on this point 



