IV.] FOUNDATION OF A THEORY OF HEREDITY. 173 



that the son would inherit an increased susceptibility of the 

 bony tissue or even of the particular bone in question. But 

 any change produced will result from the reaction of the germ- 

 cell upon changes of nutrition caused by alteration in growth at 

 the periphery, leading to some change in the size, number, or 

 arrangement of its molecular units. In the present state of our 

 knowledge there is reason for doubting whether such reaction 

 can occur at all ; but, if it can take place, at all events the quality 

 of the change in the germ-plasm can have nothing to do with 

 the quality of the acquired character, but only with the way in 

 which the general nutrition is influenced by the latter. In the 

 case of the ' Exercierknochen ' there would be practically no 

 change in the general nutrition, but if such a bony growth 

 could reach the size of a carcinoma, it is conceivable that a 

 disturbance of the general nutrition of the body might ensue. 

 Certain experiments on plants — on which Nageli showed that 

 they can be submitted to strongly varied conditions of nutrition 

 for several generations, without the production of any visible 

 hereditary change— show that the influence of nutrition upon 

 the germ-cells must be very slight, and that it may possibly 

 leave the molecular structure of the germ-plasm altogether un- 

 touched. This conclusion is also supported by comparing the 

 uncertainty of these results with the remarkable precision with 

 which heredity acts in the case of those characters which are 

 known to be transmitted. In fact, up to the present time, it has 

 never been proved that any changes in general nutrition can 

 modify the molecular structure of the germ-plasm, and far less 

 has it been rendered by any means probable that the germ-cells 

 can be affected by acquired changes which have no influence 

 on general nutrition. If we consider that each so-called pre- 

 disposition (that is, a power of reacting upon a certain stimulus 

 in a certain way, possessed by any organism or by one of its 

 parts) must be innate, and further that each acquired character 

 is only the predisposed reaction of some part of an organism 

 upon some external influence ; then we must admit that only 

 one of the causes which produce any acquired character can be 

 transmitted, the one which was present before the character 

 itself appeared, viz. the predisposition ; and we must further 

 admit that the latter arises from the germ, and that it is quite 

 immaterial to the following generation whether such predis- 



