GREAT STEPS IN EVOLUTION 83 



forms become encysted, that the encysted 

 stage gives rise to active flagellate spores, 

 and that these sink down again into amoebae. 

 The three chapters in the life-history of the 

 simplest forms are, as it were, prophecies of 

 each of the three groups — Infusorians, Rhizo- 

 pods, and Gregarines. In other words, the 

 most primitive organisms pass through a 

 cycle of three phases, one of which is ac- 

 cented by each of the three main groups of 

 Protozoa. And while each main group is 

 characterized by one dominant phase of 

 cell-life — flagellate, amoeboid or encysted — 

 there are often transient hints of other phases. 

 An infusorian may have its encysted chapter, 

 a gregarine its amoeboid stage, and a rhizopod 

 may begin as a mobile flagellate spore; for 

 each group, while accenting one phase of 

 the cycle, retains reminiscences of the others. 

 The conviction that the triple division 

 really means much, grows stronger when we 

 pass from the unicellulars to the cells that 

 compose the higher animals. For they, too, 

 may be rationally classified along the three 

 great lines. There are active ciliated or 

 flagellate cells in most animal types — the 

 flagellate cells of sponges, the "flame-cells" 

 of the lower worms, the ciliated epithelium 

 lining our air-passages, being three familiar 

 illustrations. The white blood corpuscles 



