INHERITANCE 285 



1. Modifications 



Every individual organism — a man, for instance — is a com- 

 posite not only of inherited characters, but also of modifications 

 of the soma produced by external conditions during embryonic 

 development or later. The individual's environment, food, friends, 

 enemies, the world as he finds it, on the one hand, and on the other 

 his education, work, and general reactions to this environment, 

 all have their influence on body and mind and determine to a 

 considerable extent the realization of the possibilities derived from 

 the germ — what he makes of his endowment. He acquires, let 

 us say, the strong arm of the blacksmith, the sensitive fingers of 

 the violinist, or the command of higher mathematics. In other 

 words, what he is depends on his heritage and what he does with 

 it. Now, if he does develop an inherited capacity, can he transmit 

 to his offspring this talent in a more highly developed form than 

 he himself received it? Or must his children begin at the same 

 rung of the ladder at which he started and make their own way 

 up in the world? This is the old question of the inheritance of 

 modifications, or so-called acquired characters. (Fig. 198.) 



Is the great length of the Giraffe's neck, to take a classic though 

 crude example, due to a stretching toward the branches of trees 

 during many successive generations, with the result that a slightly 

 longer neck has been gained in each generation and inherited by 

 the following? If so, it is a result of the inheritance of modifications 

 because the changes were somatic in origin. Or is the length of 

 the neck the result of the survival in each generation of those in- 

 dividuals which 'happened to be born' with longer necks and ac- 

 cordingly were better adapted to foliage conditions than those 

 which varied toward shorter necks? If so, it is not the result of 

 modifications but of changes having their origin in the germ cells. 

 (Fig. 92.) 



To-day biologists almost unanimously deny the former and 

 accept the latter interpretation — the consensus of opinion is 

 certainly that modifications, or changes in the individual body 

 due to nurture, use, and disuse, are not transmitted as such. This 

 conclusion is held chiefly because there is no positive evidence 

 of the inheritance of modifications while there is much negative 

 evidence ; and also because there is no known mechanism by which 

 a specific modification of the soma can so influence the germ com- 



