78 THE (;erm-plasm 



The basis of the process must be sought in the variability of 

 the biophors, which is followed in turn by that of the units of a 

 higher order, — the determinants and ids. These variations are 

 not by any means confined to the striictiire of the individual 

 cell, but concern primarily the finmber of cells of which an 

 organism consists. A leaf of a plant, or a bird's feather, may 

 increase considerably in size during the course of phylogeny, 

 without a change necessarily occurring in the cells which form 

 these parts. The variation will depend primarily on a multipli- 

 cation of the respective determinants. If the primitive eye of a 

 lower animal consisted of a single cell, constituting a visual rod, 

 and the power of multiplication of its determinants gradually 

 increased in the course of phylogeny, the number of identical 

 determinants which would arise during development by the 

 multiplication of the single determinant ^n the germ-plasm 

 would gradually increase so as to suffice for two cells instead of 

 one. The eye would then possess two visual rods, and if the 

 power of multiplication increased still more, a whole group of 

 visual rods would be controlled by one determinant. We are 

 unable to conjecture on what internal variations in the determi- 

 nant such an increase in the power of multiplication depends ; 

 but the fact that every individual cell in the body does not 

 possess a special determinant, while large groups of cells are 

 controlled by a single one, proves that such variations must be 

 possible. 



Such a very simple phyletic variation, resulting in the local 

 increase of the number of cells, will be followed by a further 

 variation as soon as the multiplication of the determinant of, 

 e.g., an undifferentiated sensory cell, is not confined to the later 

 stages of ontogeny, but occurs also in the germ-plasm itself; 

 that is, when the doubling of the determinant has already taken 

 place in the id of germ-plasm. For in this case the group of 

 sensory cells, which have become developed phyletically from 

 the originally single cell, will now be controlled by two deter- 

 minants, each of which can vary independently of the other, and 

 can transform the group of cells under its control. Thus one of 

 these groups might give rise to auditory cells, and the other to 

 gustatory or olfactory cells. 



Thus the increase in the differentiation of the body depends 

 primarily upon the multiplication of the determinants in the id 

 of germ-plasm, but this differentiation is only rendered complete 



