2 24 THE GERM-PLASM 



diversity of the cells in kind and of the differentiation of the 

 body. 



These two assumptions appear to me to be of equal value in 

 explaining the fact that in many of the lower plants each cell, 

 under certain circumstances, can apparently reproduce an entire 

 individual. The differences between the somatic cells are here 

 only slight ones, and are so few in number, that we might be 

 inclined to consider them as due to reactions of the same idio- 

 plasm to different kinds of influences exerted by the environ- 

 ment. Such is the case, for instance, in liverworts. But this 

 assumption ceases to be tenable as soon as the soma can be- 

 come variously differentiated, and any explanation must in the 

 first place account for this differentiation : that is to say, the 

 diversity which always exists amongst these cells and groups of 

 cells arising from the ovum must be referred to some definite 

 principle. De Vries's principle is of no use at all in this case, 

 for it only accounts for the fact that entire plants may, under 

 certain circumstances, arise from individual cells, and does not 

 even touch the main point. In fact, no one could even look 

 upon it as giving a partial solution of the problem, if differentia- 

 tion is supposed to be due to that part alone of the germ-plasm 

 always becoming active, which is required for the production of 

 the cell or organ under consideration. But the higher we ascend 

 in the organic world, the more limited does the power of pro- 

 ducing the whole from separate cells become, and the more do 

 the numerous and varied differentiations of the soma claim our 

 attention and require an explanation in the first instance. 



The presence of idioplasm in all parts containing all the 

 primary constituents does not help us in this respect ; and even 

 in attempting to explain the formation of germ-cells, it is of 

 very little use to assume that they arise from cells which, like 

 the rest, contain all the primary constituents of the species. 

 How is it that these cells, and these alone, in the entire soma of 

 the animal, give rise to germ-cells? In the lower plants the 

 fact of the differentiation of the soma is liable to be overlooked 

 or underrated, but this cannot possibly be the case as regards 

 the higher animals. 



