340 CLASSICS OF MODERN SCIENCE 

 submitted to strongly varied conditions of nutrition for several gen- 

 erations, 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 pre- 

 cision 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 afifected by 

 acquired changes which have no influence on general nutrition. If 

 we consider that each so-called predisposition (that is, a power of re- 

 acting upon a certain stimulus in a certain way, possessed by any or- 

 ganism 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 trans- 

 mitted, 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 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 hered- 

 ity, 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 as- 

 sumes 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 prod- 

 uct of the parent's body, at least as far as their essential part — the 

 specific germ-plasm — is concerned : they are rather considered as some- 

 thing which is to be placed in contrast with the tout ensemble of the 

 cells which make up the parent's body, and the germ-cells of succeed- 

 ing generations stand in a similar relation to one another as a series 

 of generations of unicellular organisms, arising by a continued process 

 of cell-division. It is true that in most cases the generations of germ- 



