10 THE AMERICAN NATURALIST [Vol. L 



data that can be collected and which must be the last ap- 

 peal for the cogency of all arguments and judgments. 

 The study of the Protista is as yet in its infancy ; groups 

 have been recognized and have received ponderous desig- 

 nations, although their very existence is yet in doubt, as 

 in the case of the so-called Chlamydozoa ; and our knowl- 

 edge of the affinities and mutual relationships of the 

 groups is still very imperfect. All attempts, therefore, 

 to trace the evolution of the Protista must be considered 

 as purely tentative at present. If I venture upon any 

 such attempt, it is to be regarded as indicating a firm be- 

 lief on my part that the evolution of the cell has taken 

 place amongst the Protista, and that its stages can be 

 traced there, rather than as a dogmatic statement that 

 the evolution has taken place in just the manner which 

 seems to me most probable. When we reflect on the ir- 

 reconcilable differences of opinion amongst zoologists 

 with regard to the origin and ancestry of vertebrates, for 

 example, we may well be cautious in accepting pedigrees 

 in Protista. 



Before, however, I can proceed to deal with my main 

 subject, it is absolutely necessary that I should define 

 clearly the sense in which I propose to use certain terms, 

 more especially the words "cell," " nucleus," "chroma- 

 tin," "protoplasm" and "cytoplasm." Unless I do so 

 my position is certain to be misunderstood, as, indeed, it 

 has been already by some of my critics. 



The term cell was applied originally by botanists to the 

 single chambers or units of the honeycombed structure 

 seen in the tissues of plants. The application of the term 

 to such structures is perfectly natural and intelligible, 

 since each such cell in its typical form is actually a closed 

 space limited by firm walls, and containing a relatively 

 large quantity of fluid cell- sap and a small quantity of 

 the slimy protoplasmic substance. When these struc- 

 tures were first discovered, the limiting membrane or wall 

 of the cell was regarded as essential, and less importance 

 was attached to its contents. With increased knowledge, 



