MODIFICATION OF THE GERM-CELLS IN MAMMALS 143 



the germ cells may be so modified as to render them incapable 

 of normal development. This might easily occur without 

 difTerentiated somatic tissues being sufficiently damaged to 

 greatly impair their usual functions. In other cases, and prob- 

 ably as a rule, the somatic tissues are also injured by any offen- 

 sive substance present in sufficient quantities to modify the 

 germ cells, and there are many reasons for believing this to be 

 the result ih several chronic human infections. 



One must not infer from these statements that the germ cells 

 are readily injured by poisons taken into the system; indeed, 

 they seem on the contrary to be protected to a remarkable degree 

 against such effects, and for this reason it is difficult to obtain a 

 substance which may be used in experimental studies on the 

 modification of the germ cells. 



Should the germ cells be modified through the action of any 

 substance, the point of particular importance is that all cells 

 arising from such a modified germ will be similarly modified, 

 since they are merely products of its division, and thus the 

 soma and germ cells of the resulting individual will deviate 

 from the normal in proportion to the degree of the primary modi- 

 fication of the cells from which it arose. Provided the change 

 is one of such a nature that the cell or its parts are unable to 

 recuperate, for example, if their specific chemical or physical 

 make up be altered, 'then not only will the generation resulting 

 from the originally modified germ cells be affected, but all future 

 generations arising from this modified germ plasm will likewise 

 be affected. 



It seems also highly probable that should such results occur, 

 the modifications to be observed in the somatic generations will 

 be of a generalized nature affecting the organism in various ways 

 so as to render its development less vigorous, its chance of sur- 

 vival less certain, and its ability to behave in a normal fashion 

 more or less hampered. In certain cases the animal might 

 really show no evident signs of its altered character. It seems to 

 us, on the other hand, that only through the very rarest chance, 

 one in possibly thousands, would any of the small number of 

 definite characters under observation happen to be modified by 

 their response to the treatment. The inheritance of coat-color, 



