THE EVOLUTION OF A BURSA 191 



ment on an analysis of a considerable mass of small facts which 

 no one disputes I claim that the modifications drawn from normal 

 anatomy on the one hand and on the other adventitious structures, 

 produced by acknowledged mechanical forces, are examples of the 

 transmission of modifications, and illustrate the mode of formation 

 of certain initial variations. In other regions where Plasto- 

 diethesis, as I conceive it, is at work in producing adapted organisms, 

 there may be included in the hyphenated area certain factors of 

 heredity, Mendelian, mutational and others, but not in this group. 

 This is merely an assertion of an opinion though I submit that there 

 is good evidence for it. Not even the hardest hearted Weismann- 

 ian, Mendelian or mutationist, and not even the biometrician can 

 refuse to this poor little province the required time and mechanical 

 forces, and, unless an opponent can offer some explanation more 

 consistent with the facts than that here offered, the proof of causation 

 is as sound as that shown in the larger one of the direction of hair. 



