GREAT STEPS IN EVOLUTION 88 



the plant the numerator is always large in pro- 

 portion to the denominator. In the animal 

 there is a relative preponderance of kata- 

 bolic processes. Thus at point after point in 

 the history of organisms the evolving Proteus 

 has had to face the alternatives of two 

 possible regimes precisely corresponding to 

 the alternatives between Plant and Animal 

 in the earliest days. 



THE CELL-CYCLE. When we take a survey 

 of a representative set of unicellular organisms 

 amoebae, foraminifers, sun-animalcules, in- 

 fusorians, gregarines, and simple algae and 

 fungi as well, we reach, almost by inspection, 

 a rough and ready tripartite classification 

 into very active and very passive forms, with 

 amoeboid forms midway. At one extreme 

 are the highly active infusorians, such as the 

 widely diffused free-living slipper-animalcules, 

 or the widely diffused parasitic trypanosomes 

 (one of which causes sleeping-sickness); at the 

 opposite extreme are quiescent forms, in which 

 the life seems to sleep; between the two 

 the amoeboid forms have evolved along a 

 via media a compromise between extreme 

 activity and extreme passivity. 



If we go deeper than mere inspection and 

 study the life-history of the very simplest 

 forms, such as some of the primitive Proteo- 



