FOUNDATION OF A THEORY OF HEREDITY. 183 



germ-plasm of a worm is qualitatively different from that of Am- 

 phioxus, a frog, or a mammal. But if such phyletic stages occur 

 crowded together in the ontogeny of a single species, they are said to 

 be based upon different ' conditions of tension and movement ' of one 

 and the same idioplasm ! It seems to me to be necessary to con- 

 clude that if the idioplasm, in the course of phyletic development, 

 undergoes any alteration in specific constitution, such alterations 

 must also take place in ontogeny ; so far at least as the phyletic 

 stages are repeated. Either the whole phyletic development is 

 based upon different ' conditions of tension and movement,' or if 

 this as I believe is impossible, the stages of ontogeny must 

 be based upon qualitative alterations in the idioplasm. 



Involuntarily the question arises how is it that such an acute 

 thinker fails to perceive this contradiction ? But the answer is 

 not far to seek, and Nageli himself indicates it when he adds these 

 words to the sentence quoted above : ' It follows therefore that 

 if a cell is detached as a germ-cell in any stage of ontogenetic 

 development, and from any part of the organism, such a cell will 

 contain all the hereditary tendencies of the parent individual.' 

 In other words, if we are restricted to different 'conditions of 

 tension and movement ' as an explanation, it seems to follow as a 

 matter of course that the idioplasm can re-assume its original 

 condition, and therefore that the idioplasm of any cell in the 

 body can again become the idioplasm of the germ-cell ; for this to 

 take place it is only necessary that the greater tension should 

 become the less, or vice versa. But if we admit a real change 

 in constitution, then the backward development of the idio- 

 plasm of the cells of the body into germ-cells appears to be 

 very far from a matter of course, and he who assumes it must 

 bring forward weighty reasons. Nageli does not produce such 

 reasons, but considers the metamorphosis of the idioplasm in on- 

 togeny as mere differences in the ' conditions of tension and move- 

 ment.' This phrase covers the weak part of his theory ; and I 

 look upon it as a valuable proof that ^Jageli has also felt that the 

 phenomena of heredity can only find their explanation in the 

 hypothesis of the continuity of the germ-plasm ; for his phrase is 

 only capable of obscuring the question as to how the idioplasm 

 of the cells of the body can be re-transformed into the idioplasm of 

 germ-cells. 



