112 Theory of the Germ-Plasm 



The group of the pseudo-somatic tracks may be chosen as 

 an illustration, 23 and I shall come back to further instances 

 in the last paragraph of this Section. 



Therefore germ-cells and somatic cells do not present 

 any qualitative contrast in the plant kingdom. They are the 

 extremes of a long line of quantitative differences. This 

 law I regard as one of the most important results of the 

 consideration of vegetative cell-pedigrees. Sachs, Stras- 

 burger, and others, have pointed out the importance of 

 this law, and it seems to me that the foregoing compre- 

 hensive descriptions ought to contribute in causing the 

 conviction of its correctness to become general. 



On the distinction between germ-cells and somatic 

 cells Weismann founded his theory of the germ-plasm. 

 The latter must, therefore, be present in all the germ-cells. 

 But according to Weismann, it is only in these that it needs 

 to be retained, while it must be lacking in the somatic 

 cells, because they cannot reproduce the species. They 

 are limited to the unfolding of a limited number of hered- 

 itary units, and thus need only that portion of the germ- 

 plasm requisite thereto. These considerations induced 

 Weismann to regard the germ-plasm as a special sub- 

 stance, which, in contrast to the remaining or somatic 

 plasm, is the vehicle of heredity. 



In the first part we have seen how the theory of a germ- 

 plasm fails us in the explanation of the differentiation of 

 organs. There the assumption of one substance is not 

 sufficient ; special material bearers of the individual hered- 

 itary characters, the so-called pangens, were necessary for 

 the explanation. Their assumption, however, rendered 

 the assumption of the germ-plasm with its consequences, 

 superfluous. 



23 Cf. Section 6. p. 100. 



