380 THE EVOLUTION THEOEY 



active is similar to that suggested by De Vries in regard to his 

 ' Pangens,' very minute vital particles which play a determining part 

 in his 'pangen theory/ similar to that filled by the determinants 

 in my germ-plasm theory. It seems to me that the determinants 

 must ultimately break up into the smallest vital elements of which 

 they are composed, the biophors, and that these migrate through the 

 nuclear membrane into the cell-substance. But there a struggle for 

 food and space must take place between the protoplasmic elements 

 already present and the newcomers, and this gives rise to a more 

 or less marked modification of the cell-structure. 



It might be supposed that the structure of these biophors corre- 

 sponded in advance to certain constituent parts of the cell, that there 

 were, for instance, muscle biophors, which make the muscle what 

 it is, or that the plant-cells acquired their chlorophyll-making 

 organs through chlorophyll biophors. De Vries gave expression 

 to this view in his ' pangen theory,' and I confess that at the time 

 there seemed to me much to be said for it, but I am now doubtful 

 whether its general applicability can be admitted. In the first place, 

 it does not seem to me theoretically necessary to assume that the 

 particles which migrate into the cell-bodies should themselves be 

 chlorophyll or muscle particles ; they may quite well be only the 

 architects of these, that is to say, particles which by their co-operation 

 with the elements already present in the cell- body give rise to chloro- 

 phyll or muscle substance. As we are as yet unacquainted with the 

 forces which dominate these smallest vital particles, as well as the 

 processes which lead to the histological differentiation of the cells, 

 it is useless in the meantime to make any further hypotheses in regard 

 to them. But in any case the biophors which transform the general 

 character of the embryonic cells into the specific character of a parti- 

 cular tissue-cell must themselves possess a specific structure different 

 from that of other biophors, for they must keep up the continuity 

 of the structures handed on from ancestors, chlorophyll and muscle- 

 substance and the like, since we cannot assume that these structures, 

 so peculiar and so complex in their chemical and physical constitution, 

 are formed afresh, so to speak, by spontaneous generation in each new 

 being, as De Vries has very rightly emphasized. A specific biophor, 

 for instance, of muscle substance will produce this substance as soon 

 as it has found its way into the appropriate cell-body, even though 

 it may not be a contractile element itself. 



To this must be added that the structure of the body and the 

 distinctive features of an organism do not depend merely on the 

 histological differentiation of the cells, but quite as much on their 



