8 



THE AMERICAN NATURALIST 



[ Vol. L 



of life. So long, however, as the attention of cytologists 

 is confined to the study of the cells building up the bodies 

 of the higher animals and plants, they are not brought 

 face to face with the stages of evolution of the cell, but are 

 confronted only with the cell as a finished and perfected 

 product of evolution, that is to say, with cells which, al- 

 though they may show infinite variation in subordinate 

 points of structure and activity, are nevertheless so fun- 

 damentally of one type that their plan of structure and 

 mode of reproduction by division can be described in gen- 

 eral terms once and for all in the first chapter of a bio- 

 logical text-book or in the opening lecture of a course of 

 elementary biology. 



One of the most striking features of the general trend 

 of biological investigation during the last two decades 

 has been the attention paid to the Protista, that vast as- 

 semblage of living beings invisible, with few exceptions, 

 to the unassisted human vision and in some cases minute 

 beyond the range of the most powerful microscopes of 

 to-day. The study of the Protista has received in recent 

 years a great stimulus from the discovery of the im- 

 portance of some of the parasitic forms as invaders of 

 the bodies of men and animals and causers of diseases 

 often of a deadly nature ; it has, however, yielded at the 

 same time results of the utmost importance for general 

 scientific knowledge and theory. The morphological 

 characteristic of the Protista, speaking generally, is that 

 the body of the individual does not attain to a higher de- 

 gree of organization than that of the single cell. The ex- 

 ploitation, if I may use the term, of the Protista, though 

 still in its initial stages, has already shown that it is 

 amongst these organisms that we have to seek for the 

 forms which indicate the evolution of the cell, both those 

 lines of descent which lead on to the cell as seen in the 

 Metazoa and Metaphyta, as well as other lines leading in 

 directions altogether divergent from the typical cell of 

 the text-book. We find in the Protista every possible condi- 

 tion of structural differentiation and elaboration, from cells 



