J 86 CONTINUITY OF THE GERM-PLASM AS THE [IV. 



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 conclude 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 move- 

 ment,' 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 explana- 

 tion, it seems to follow as a matter of course that the idioplasm 

 can re-assume its original condition, and therefore that the idio- 

 plasm 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 idioplasm 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 weight}' reasons. 

 Nageli does not produce such reasons, but considers the 

 metamorphosis of the idioplasm in ontogeny as mere differ- 

 ences in the ' conditions of tension and movement.' This 

 phrase covers the weak part of his theory ; and I look upon it 

 as a valuable proof that Nageli 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. 



I am of the opinion that the idioplasm cannot be re-trans- 



