IV.] FOUNDATION OF A THEORY OF HEREDITY. 171 



from which the explanation of these phenomena can be at- 

 tempted. 



It is true that this theory also meets with difficulties, for it 

 seems to be unable to do justice to a certain class of pheno- 

 mena, viz. the transmission of so-called acquired characters. 

 I therefore gave immediate and special attention to this point 

 in my first publication on heredity^, and I believe that I have 

 shown that the hypothesis of the transmission of acquired cha- 

 racters — up to that time generally accepted— is, to say the least, 

 very far from being proved, and that entire classes of facts 

 which have been interpreted under this hypothesis may be 

 quite as well interpreted otherwise, while in many cases they 

 must be explained differently. I have shown that there is no 

 ascertained fact, which, at least up to the present time, remains 

 in irrevocable conflict with the hypothesis of the continuit}^ of 

 the germ-plasm ; and I do not know any reason why I should 

 modify this opinion to-day, for I have not heard of any objection 

 which appears to be feasible. E. Roth ^ has objected that in 

 pathology we everywhere meet with the fact that acquired local 

 disease may be transmitted to the offspring as a predisposition ; 

 but all such cases are exposed to the serious criticism that the 

 very point that first needs to be placed on a secure footing is 

 incapable of proof, viz. the hypothesis that the causes which 

 in each particular case led to the predisposition were really 

 acquired. It is not my intention, on the present occasion, to 

 enter fully into the question of acquired characters ; I hope to 

 be able to consider the subject in greater detail at a future date. 

 But in the meantime I should wish to point out that we ought, 

 above all, to be clear as to what we really mean by the ex- 

 pression ' acquired character.' An organism cannot acquire any- 

 thing unless it already possesses the predisposition to acquire 

 it : acquired characters are therefore, no more than local or 

 sometimes general variations which arise under the stimulus 

 provided by certain external influences. If by the long-con- 

 tinued handling of a rifle, the so-called ' Exercierknochen ' (a 

 bony growth caused by the pressure of the weapon in drilling) 



' Weismann, ' Ueber die Vererbung.' Jena, 1883 ; translated in the 

 present volume as the second essay ' On Heredity.' 



■" E. Roth, ' Die Thatsachen der Vererbung.' 2. Autl., Beriin, 1885, 

 p. 14. 



