82 EVOLUTION 



at point after point in the history of organ- 

 isms the evolving Proteus has had to face 

 the alternatives of two possible regimes 

 precisely corresponding to the alternative 

 between Plant and Animal in the earliest 

 days. 



The Cell-Cycle. — When we take a sur- 

 vey of a representative set of unicellular 

 organisms — amoebae, foraminifers, sun-ani- 

 malcules, infusorians, gregarines, and simple 

 algse and fungi as well, we reach, almost by 

 inspection, a rough and ready tripartite 

 classification into very active and very pas- 

 sive 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 com- 

 promise between extreme activity and ex- 

 treme 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- 

 myxa and Myxomycetes, we get a new light 

 on our classification. For in these life- 

 histories we find, for instance, that amoeboid 



