Experimental Transmission of Modifications 323 



maternal and paternal contributions to remain distinct, as 

 is shown by the segregations in the progeny of hybrids. 

 Hybridizing brings into the germ plasm new combina- 

 tions, but the alternative traits do not affect one another. 

 The genes, or determiners, or factors present in the 

 germ cells retain their individuality from generation to 

 generation. 



With negative results from experiments in grafting and 

 in genetics, it is possible at most to conclude that the trans- 

 mission of acquired characters is " not proven." While the 

 burden of proof rests upon those who believe that there is 

 such transmission, those in possession of the negative evi- 

 dence must not be too positive in their denials. It should 

 be recognized, however, that such positive evidence as has 

 been offered can quite as well be explained by known princi- 

 ples of genetics as by the Lamarckian principles. 



Experimental Transmission of Modifications 



In more recent times biologists have found it necessary 

 to discriminate, in considering this problem, between a pos- 

 sible effect upon the germ plasm, and the possible transmis- 

 sion through the germ of an effect upon the body. This is 

 an important distinction, for we have already seen (page 

 164) that radium and chemical processes can modify the 

 germ cells. In such modifications of the germs, however, 

 we never see the reproduction of a character that has been 

 induced in the parent organism. Alcoholism, for example, 

 has resulted in defective offspring: but the effect in the off- 

 spring, conceded to result from the alcoholizing of the 

 parents, is not the same kind of effect as is produced in the 

 parent by the alcohol. Aside from the fact that in these 

 experiments the results were always more or less injurious 

 rather than adaptive or useful, they are of a distinct order 

 in the germ and in the soma. These facts have given rise to 

 the theory of induced changes in the germ cells. The prob- 

 lem then takes the form of asking whether changes induced 



