SUPPOSED TRANSMISSION OF ACQUIRED CHARACTERS 393 



Somatogenic variations may be classified according to their 

 origin into tliree categories, — \\i., injuries, functional varia- 

 tions, Tiwd variations depending on the ?,o-c2i\\&d ' influences of 

 environment,' — which include mainly climatic variations. 



The hereditary transmission of any of these three kinds of 

 somatogenic variations could be accounted for theoretically only 

 bv the assumption that that part of the soma which had been 

 changed by external influences, could modify the germ-plasm 

 contained in the germ-cells of the same individual, so that its 

 offspring would, from the germ onwards, undergo similar varia- 

 tions to those which had been acquired by the action of external 

 influences on the parental part in question. 



As far as I can see, there are only two ways in which such a 

 variation could conceivably occur in the germ-plasm in con- 

 sequence of a corresponding somatic variation. We should 

 either have to assume the presence in all parts of the body of 

 definite tracks along which each somatogenic variation might 

 be transferred to the germ-cells, in the germ-plasm of which it 

 would produce a corresponding change ; or else that gemmules, 

 such as Darwin supposed to exist, are given off" from every 

 somatic cell and are conveyed to the germ-cells, — either 

 through the vascular system, when one exists, — or by some 

 other means, and that they must then penetrate into these 

 cells, and become incorporated in their germ-plasm. Thus 

 either the presence of hypothetical tracks along which a 

 modifying, though totally inconceivable, influence might be 

 transferred to the germ-cells, or else the discharge of material 

 particles from the modified organ, must take part in the for- 

 mation of the germ-plasm : there is no third way out of the 

 difficulty. 



Both these hypotheses have already been used to explain 

 the supposed transmission of somatogenic variations, — the 

 former, it is true, only in vague allusions hinting at ' nerve- 

 influences,' which are supposed to start from the modified part, 

 and to produce a corresponding alteration in the hereditary sub- 

 stance of the germ-cells. But no one has yet ventured to state 

 more precisely how nerve-excitation can modify the germ-plasm 

 materially^ and in accordance ivitli the somatic variation. It 

 would probably be useless even to expect an answer to the 

 question as to how a part, such as a muscle, enlarged by functional 

 hypertrophy, is capable of producing a specific nervous current 



