24 THE BIOLOGICAL PROBLEM OF TO-DA Y 



which I shall not discuss here, regards the germ- 

 plasm as being still more complicated, and consist- 

 ing of many, sometimes more than a hundred, 

 ancestral plasms or ids, which have been derived 

 from near or distant ancestors, the peculiarities of 

 whose structures they retain, and may at some time 

 actually produce (explanation of atavism). 



But how does this fabric, endowed with an archi- 

 tecture so complicated, actually produce the de- 

 velopment of the adult from the egg ? The natural 

 mechanism for this purpose is cell division and 

 nuclear division. 



According to Weismann's supposition a sup- 

 position which forms, as we shall see, a chief 

 corner stone of his system there are two kinds of 

 nuclear division, the difference between which has 

 not been observed, but is a corollary from the 

 difference between their results. The one kind is 

 denoted as integral, or doubling division ; the other 

 as differential, or differentiating division. The first 

 method has only an incidental importance in Weis- 

 mann's hypothesis : it consists of the doubling by 

 growth of the rudiments, and of a perfectly fair 

 division of them between the half-chromosomes; 

 it occurs in tissues-cells, where parent-cells divide 

 into daughter-cells exactly similar to each other 

 and to their parents. 



On the other hand, in differentiating division the 

 rudiments become irregularly grouped during their 

 growth ; consequently, on division of the ids, which 

 are composed of determinants, totally different com- 

 binations of the determinants are included in the 



