No. 590] 



THE EVOLUTION OF THE CELL 



109 



ing for a time, and disappearing again. This analogy be- 

 tween the chromatin of cells and the germ-plasm of multi- 

 cellular organisms becomes still more marked when we 

 find that in many Protozoa the chromatin may undergo a 

 serialization into generative and trophic chromatin, the 

 former destined to persist from one life-cycle to another, 

 the latter destined to control cell-activities merely during 

 one cycle, without persisting into the next. The differen- 

 tiation of generative and trophic chromatin is now well 

 known to occur in many Protozoa, and in its most extreme 

 form, as seen in the Infusoria, it is expressed in occur- 

 rence of two distinct nuclei in the cell-body. 



To recapitulate my argument in the briefest form ; the 

 chromatinic constituents of the cell contrast with all the 

 other constituents in at least three points : physiological 

 predominance, especially in constructive metabolism ; spe- 

 cific individualization; and permanence in the sense of 

 potential biological immortality. Any of these three 

 points, taken by itself, is sufficient to confer a peculiar dis- 

 tinction to say the least, on the chromatin-bodies ; but 

 taken in combination they appear to me to furnish over- 

 whelming evidence for regarding the chromatin-elements 

 as the primary and essential constituents of living or- 

 ganisms, and as representing that part of a living body 

 of any kind which can be followed by the imagination, in 

 the reverse direction of the propagative series, back to the 

 very starting-point of the evolution of living beings. 



In the attempt to form an idea as to what the earliest 

 type of living being was like, in the first place, and as to 

 how the earliest steps in its evolution and differentiation 

 came about, in the second place, we have to exercise the 

 constructive faculty of the imagination guided by such few 

 data as we possess. It is not to be expected, therefore, 

 that agreement can be hoped for in such speculations ; it 

 would indeed be very undesirable, in the interests of 

 science, that there should be no conflict of opinion in 

 theories which, by their very nature, are beyond any pos- 

 sibility of direct verification at the present time. The 



