GREAT STEPS IN ORGANIC EVOLUTION 857 



Protozoa as single cells. They are rather non-cellular than uni- 

 cellular. They are organisms, often very intricate, often with elabo- 

 rate beauty, often very adventurous, which have elected to do 

 business on a bodiless basis. To what are they comparable but to 

 the germ-cells of bodied animals — organisms telescoped-down to 

 a unit expression ? With this proviso, one may continue to speak of 

 the Protozoa as single cells. 



One of the great eras in Organic Evolution was that during which 

 the forms of unicellular life became differentiated, and the path of 

 differentiation they have not ceased to pursue. In protean variety 

 there arose among the Protozoa and the Protophyta many different 

 types, often with little in common save that they were single cells 

 complete in themselves. 



THE CELL CYCLE. — When we avoid the complexity involved 

 in a systematic classification of the Protozoa, and try not to lose 

 sight of the wood in the trees, we may discern three main pathways 

 in the evolution of the group, of which a general interpretation 

 was offered many years ago by one of us, in terms of the conception 

 of "the cell-cycle". 



Some of the simplest Protozoa, or organisms antecedent to the 

 Protozoa, namely, the Proteomyxa and Myxomycetes, pass through 

 a cycle of phases. A flagellate spore may sink down into an amoeboid 

 unit ; a number of amoeboid units may coalesce to form a composite 

 amoeboid Plasmodium; a part of the plasmodium may encyst and 

 divide into minute units which escape as flagellate spores. Here, 

 then, is a cell-cycle showing a passage from flagellate to amoeboid, 

 from amoeboid to encysted, from encysted to flagellate again, the 

 plasmodial phase being interpolated as an additional chapter. This 

 is the primitive cell-cycle, and there are many illustrations of it. 

 The Protozoa show a tripartite grouping — the very active, pre- 

 dominantly katabolic Infusorians, the very sluggish, preponder- 

 atingly anabolic Sporozoa (in particular the Gregarinids), and the 

 more or less amoeboid Rhizopods between — on a via media between 

 the two extremes. 



The unifying idea thus suggested is simply that the Infusorians, 

 Rhizopods, and Sporozoa, the three great classes of Protozoa, are, 

 as it were, the respective accentuations of the flagellate, amoeboid, 

 and encysted phases in the life-history of the simplest. Moreover, 

 each often shows traces of other phases besides that which it mainly 

 represents. Thus an Amoeba often passes into an encysted state, a 

 young Radiolarian may be flagellate, a young Sporozoon may be 

 flagellate or amoeboid — partial expressions of the primitive tendency 

 to a cell-cycle. 



To adduce as an objection to this idea the fact that our once 

 single class (or order!) of Infusorians has been, with increasing 



