Theory of the Germ-Plasm 111 



This doctrine, and the hypothesis of the ancestral 

 plasms which is based on it, have already been critically 

 reviewed in the first Part. I have there (p. 56) also 

 pointed out the fact that, in the face of a detailed consid- 

 eration of cell-pedigrees, it cannot be maintained. Now 

 that we have become more familiar with these latter, it 

 must be our task to endeavor to establish this claim. 



The true significance of the difference between the 

 germ-tracks and the somatic cells can be correctly judged 

 only when glancing over the whole richness of the ramifi- 

 cations of a highly differentiated cell-pedigree. And it is 

 only in plants that this differentiation reaches its highest 

 degree. Numerous intermediate forms lead here, with 

 almost imperceptible transitions, from the main germ- 

 track to the somatic tracks. 



For this very reason I have laid particular stress on 

 the discussion of the secondary germ-tracks. They are 

 wanting in the higher animals. In the plant kingdom they 

 are present in all gradations. I have not attempted to 

 draw a sharp line of demarcation between them and the 

 main germ-tracks ; such an attempt would be thwarted by 

 the same difficulties which make impossible the exact lim- 

 itation of the concept "individual." We must be satisfied 

 here with an arbitrary limit, and choose the one that seems 

 most convenient. 



The difficulties that confront us on the border-line be- 

 tween secondary germ-tracks and somatic tracks are of 

 a different nature. Here they are due to the incomplete- 

 ness of our knowledge. I call those tracks that do not 

 lead to a propagation of the species somatic. But many 

 cells, many a tissue-complex which, on this ground, we 

 now call somatic, will prove itself, on further experimen- 

 tation, to be provided with the power of reproduction. 



