336 CLASSICS OF MODERN SCIENCE 

 But although I am of the opinion that the whole foundation of the 

 theory of Pangenesis, however it may be modified, must be abandoned, 

 I think, nevertheless, its author deserves great credit, and that its 

 production has been one of those indirect roads along which science has 

 been compelled to travel in order to arrive at the truth. Pangenesis 

 is a modern revival of the oldest theory of heredity, that of Democ- 

 ritus, according to which the sperm is secreted from all parts of the 

 body of both sexes during copulation, and is animated by a bodily 

 force ; according to this theory also, the sperm from each part of the 

 body reproduces the same part. 



If, according to the received physiological and morphological ideas 

 of the day, it is impossible to imagine that gemmules produced by each 

 cell of the organism are at all times to be found in all parts of the body, 

 and furthermore that these gemmules are collected in the sexual cells, 

 which are then able to reproduce again in a certain order each separate 

 cell of the organism, so that each sexual cell is capable of developing 

 into the likeness of the parent body ; if all this is inconceivable, we 

 must inquire for some other way in which we can arrive at a founda- 

 tion for the true understanding of heredity. My present task is not 

 to deal with the whole question of heredity, but only with the single 

 although fundamental question — *'How is it that a single cell of the 

 body can contain within itself all the hereditary tendencies of the 

 whole organism?" I am here leaving out of account the further 

 question as to the forces and the mechanism by which these tenden- 

 cies are developed in the building-up of the organism. On this ac- 

 count I abstain from considering at present the views of Nageli, for 

 as will be shown later on, they only slightly touch this fundamental 

 question, although they may certainly claim to be of the highest im- 

 portance with respect to the further question alluded to above. 



Now if it is impossible for the germ-cell to be, as it were, an extract 

 of the whole body, and for all the cells of the organism to dispatch 

 small particles to the germ-cells, from which the latter derive their 

 power of heredity; then there remain, as it seems to me, only two 

 other possible, physiologically conceivable, theories as to the origin of 

 germ-cells, manifesting such powers as we know they possess. Either 

 the substance of the parent germ-cell is capable of undergoing a series 

 of changes which, after the building-up of a new individual leads back 

 again to identical germ-cells ; or the germ-cells are not derived at all, 



