TRANSMISSIBILITY OF FUNCTIONAL MODIFICATIONS 111 



be taken to heai-t, especially by all those who speak of simplicity in 

 reference to the germ-plasm. So much at least is certain : If there 

 were any inheritance of functional modifications, we should have 

 another proof that the germ-plasm is composed of determinants, for 

 without them there could be no possibility that the ' experiences ' of 

 an individual organ would be transmitted to the germ in the way that 

 the Lamarckian principle implies. Something, and that something 

 material, must be modified in the germ-plasm if the vigorous use of 

 a group of muscles, or of a gland, or of a nerve-cell, is to be com- 

 municated to the germ, and not to the whole germ-plasm, but only 

 to so much of it as is necessary to cause variation in the correspond- 

 ing group of cells in the child. It may perhaps be said that this still 

 does not necessitate the assumption of special determinants for these 

 cell-groups, and that one might, with Herbert Spencer, conceive of the 

 germ-plasm as consisting of homogeneous units which vary in the 

 development in accordance with the diverse regularly alternating 

 influences to which it is exposed from step to step, and that, therefore, 

 in each of these units of very complex structure only a single molecule, 

 or perhaps only a single atom, would need to varj' in order that, in 

 the course of development, the resulting cell-group should appear in 

 the rudiment in somewhat altered strength. 



But I do not believe that a chemical molecule, still less an atom, 

 is sufficient for this, for reasons which I have already stated — yet we 

 need not go into this now, but rather deduce the consequences of this 

 admission. It f oUows that the ' unit ' is made up of numerous ' mole- 

 cules ' or ' atoms,' of Which each, by dint of changes it has undergone, 

 causes particular parts of the body to vary in a definite manner; 

 in other words, we have here again a theory of determinants, only 

 they are on a much smaller scale, since each invisible little ^'ital 

 particle or ' unit ' contains all the determinants within itself, while in 

 my theory it is only the id, that is, the visible chromosome, which 

 includes the determinant complex. Such a theory would be far from 

 a simplification of mine, it would rather complicate it enormously, 

 and that without anything being gained. At most it would be made 

 more evident how inconceivably complex the nerve-paths must be 

 which lead from the part that has been modified by exercise to the 

 germ-plasm, and must also lead to all the innumerable ' molecules or 

 atoms ' of the individual ' units.' But even on my theory of the com- 

 position of the ids as aggregates of living determinants, such nervous 

 transmission of qualities would be a monstrosity which no one would 

 accept, and I think on this account that my argument as to the 

 impossibility of conceiving of the transmission of the modifications of 



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