10 THE GERM-PLASM 



tion, I shall postpone any further consideration of it to the 

 section which treats of this subject. 



In the following year, Nageli's ' Mechanico-physiological 

 Theory of Descent'* appeared. This book, which abounds 

 in ingenious deductions and important suggestions, doubtless 

 exercised a great influence on the views of that time. Its im- 

 portance cannot be denied, even if, as I believe to be the case, 

 only a small portion of its theoretical propositions can be 

 retained. Many as are the fruitful ideas and anticipation of 

 facts afterwards proved which we owe to Nageli, his own theory 

 of heredity has already become untenable. For this reason, 

 and also because the theory is so well-known, I will not describe 

 it fully here, but will only refer to the remarks which I made 

 on the subject some years ago,t and to the recent detailed criti- 

 cism by Wiesner.:}: Although I do not consider that Nageli's 

 hypothesis leads us towards a true theory of heredity, it never- 

 theless contains an important suggestion, that of the idioplasm, 

 which gives us a further insight into the problem. I had already 

 assumed the existence of a special reproductive substance — the 

 germ-plasm — on the changes of which development depends, 

 while heredity rests on its continuity : and now Nageli inde- 

 pendently postulated a special reproductive substance, an ^ Anla- 

 genplasma' or "idioplasm,' which although much smaller in bulk 

 than the rest of the living substance of the body — the tropho- 

 plasm (• Ernahrungsplasma ') — determines the detailed construc- 

 tion of the latter. The correctness of this conjecture has not as 

 yet, so far as I know, been disputed, although it was very soon 

 shown that Nageli was wrong as regards the form in which he 

 imagined the idioplasm to exist. He represented it as consist- 

 ing of very fine parallel fibres which, by uniting into bundles and 

 crossing each other so as to form a network, traverse the sub- 

 stance of the cell, and being continuous from cell to cell, pervade 

 the whole body as a connected network. 



At the time when Nageli's book appeared, it was already 

 suspected that the reproductive substance is not contained in 



* C. V. Nageli, ' Mechanisch-physiologische Theorie der Abstam- 

 mungslehre,' Miinchen and Leipzig, 1884. 



t Vide 'The Continuity of the Germ-plasm,' 1885 (pp. 180 et seq., 

 192, &c.), 



X Julius Wiesner, ' Die Elementarstructur und das Wachsthum der 

 lebenden Substanz,' Wien, 1892. 



