GEXEOLOGICAL. 413 



from early years onwards a muscle may pass into a 

 state of atrophy, through prolonged exercise another 

 may become exaggerated, and the modifications in 

 either case may last a lifetime. Endless examples 

 might be given. 



But to understand the matter more clearly we 

 must contrast " modifications " due to " nurture " 

 with " variations " due to " nature." When we 

 compare living creatures of the same kind, children 

 with parents, brother with brother, neighbour with 

 neighbour, native with foreigner, we recognise that 

 there are many differences between them, though they 

 all fall within the range which we call " the same 

 species." To begin with, we call these the observed 

 differences between individuals. As we come to 

 analyse them, however, we discern that a number are 

 definitely associated with particular functions and 

 surrounding influences. They may not be hinted at 

 in the young forms, but they begin to appear when 

 the particular conditions begin to operate. They 

 can be definitely related to some alteration or dif- 

 ference in environment or in function, and they are 

 usually exhibited in some degree by all organisms 

 of the same kind which are subjected to the same 

 change of conditions. These we call " modifica- 

 tions " or acquired characters. Now when we sub- 

 tract from the total of observed differences the modi- 

 fications which we have detected, there remain a 

 number of differences which we call " variations." 

 We cannot causally relate them to differences in habit 

 or surroundings, they are often hinted at even before 

 birth, and they are not alike even among forms whose 

 conditions of life seem absolutely uniform. We 

 suppose that they have an origin in changes of the 

 germinal material before or after fertilisation; we 



