108 A NEW THEORY OF EVOLUTION. 



the category to which a difference belongs. 

 In the human species, for example, it is 

 sometimes impracticable to distinguish be- 

 tween variety of expression and the specific 

 variation that indicates a distinct race. 



Variety of Expression. 



We are all familiar with the infinite var- 

 iety of expression, both in face and figure, 

 among individuals of the same human race, 

 and we all recognise that a particular ex- 

 pression may be more or less hereditary in 

 a family. We also know that a family ex- 

 pression may be more or less affected by the 

 conditions of existence of the individual. 

 But common-sense tells us that these differ- 

 ences of expression do not indicate any ten- 

 dency towards specific variation. Careful 

 observers may also detect similar differences 

 of expression in the same breed of domestic 

 animals, and even of wild species. But, as 

 Darwin points out, the differences among 

 domestic animals of the same race, and espe- 

 cially those that arise under what is called 

 " selective breeding," are much greater than 

 among wild species. 



When, however, the differences between 

 the best and worst specimens of any domestic 



