172 CONTINUITY OF THE GERM-PLASM AS THE [IV. 



is developed, such a result depends upon the fact that the 

 bone in question, Hke every other bone, contains within itself 

 a predisposition to react upon certain mechanical stimuli, by 

 growth in a certain direction and to a certain extent. The pre- 

 disposition towards an ' Exercierknochen ' is therefore already 

 present, or else the growth could not be formed ; and the same 

 reasoning applies to all other ' acquired characters.' 



Nothing can arise in an organism unless the predisposition to 

 it is pre-existent, for every acquired character is simply the 

 reaction of the organism upon a certain stimulus. Hence 

 I should never have thought of asserting that predispositions 

 cannot be transmitted, as E. Roth appears to believe. For 

 instance, I freely admit that the predisposition to an ' Exercier- 

 knochen ' varies, and that a strongly marked predisposition 

 may be transmitted from father to son, in the form of bony 

 tissue with a more susceptible constitution. But I should 

 deny that the son could develope an ' Exercierknochen ' with- 

 out having drilled, or that, after having drilled, he could 

 develope it more easily than his father, on account of the 

 drilling through which the latter first acquired it. I believe 

 that this is as impossible as that the leaf of an oak should pro- 

 duce a gall, without having been pierced by a gall-producing 

 insect, as a result of the thousands of antecedent generations of 

 oaks which have been pierced by such insects, and have thus 

 ' acquired ' the power of producing galls. I am also far from 

 asserting that the germ-plasm — which, as I hold, is transmitted 

 as the basis of heredity from one generation to another — is ab- 

 solutely unchangeable or totally uninfluenced by forces residing 

 in the organism within which it is transformed into germ-cells. 

 I am also compelled to admit that it is conceivable that organ- 

 isms may exert a modifying influence upon their germ-cells, 

 and even that such a process is to a certain extent inevitable. 

 The nutrition and growth of the individual must exercise some 

 influence upon its germ-cells; but in the first place this in- 

 fluence must be extremely slight, and in the second place it 

 cannot act in the manner in which it is usually assumed that it 

 takes place. A change of growth at the periphery of an organ- 

 ism, as in the case of an ' Exercierknochen,' can never cause 

 such a change in the molecular structure of the germ-plasm as 

 would augment the predisposition to an ' Exercierknochen,' so 



