FOUNDATION OF A THEORY OF HEREDITY. 171 



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

 the general nutrition of the body might ensue. Certain experi- 

 ments on plants in which Nageli showed that they can be sub- 

 mitted to strongly varied conditions of nutrition for several genera- 

 tions, 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 untouched. 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 

 predisposition (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 follow- 

 ing generation whether such predisposition comes into operation or 

 not. The continuity of the germ-plasm is amply sufficient to 

 account for such a phenomenon, and I do not believe that any 

 objection to my hypothesis, founded upon the actually observed 

 phenomena of heredity, will be found to hold. If it be accepted, 

 many facts will appear in a light different from that which has been 

 cast upon them by the hypothesis which has been hitherto received, 

 a hypothesis which assumes that the organism produces germ- 

 cells afresh, again and again, and that it produces them entirely 

 from its own substance. Under the former theory the germ-cells 

 are no longer looked upon as the product of the parent's body, at 

 least as far as their essential part the specific germ -plasm is 

 concerned : they are rather considered as something which is to be 

 placed in contrast with the tout ensemble of the cells which make 



