EVOLUTION 939 



EVOLUTION OF PROTOZOA.— Keeping in mind the idea of 

 the Cell-Cycle {q.v.), we see that the phylum of the Protozoa, which 

 includes half a dozen or so classes, needs to be envisaged, from our 

 physiological-evolutionary point of view, as illustrating the varying 

 preponderance of each of these three chief possibilities of cell-hfe 

 — great activity such as the Ciliate and Flagellate Infusorians 

 display, great sluggishness as characteristic of the encysted Sporozoa, 

 and an intermediate line of life, neither very active nor very passive, 

 but in a general way more or less amoeboid, that of the Rhizopods 

 or Sarcodina. What objections can be raised against this way of 

 looking at the facts ? 



Consider it more fuUy. It may be pointed out, as an objection, 

 that in the class Proteomyxa, which are in many ways primitive, 

 even in sometimes lacking a nucleus (cf. HaeckeFs "Monera"), (i) 

 there may be flagellate spores which may become amoeboid; (2) 

 several small amoeboid units ("amoebulae") may fuse into a larger 

 mass as in Vampyrella and Protomyxa; (3) and that a cyst is formed 

 around one unit or more, whether as a protection against drought 

 and the like, or as a preliminary to spore-formation. Thus in this 

 class of Proteomyxa, or even in a single type, like Protomyxa, there 

 may be flagellate, amoeboid, and encysted phases. But this is what 

 might be expected among primitive organisms, which have not yet 

 taken predominantly to any one line of cell-life. Nothing could be 

 wished better to confirm us in the conception of the cell-cycle. 

 And this cell-cycle theory expressly recognises Plasmodium forma- 

 tion, as a multiple conjugation, and thus reappearing in various 

 forms not necessarily related. 



Secondly, it may be pointed out by the critic that the suggested 

 physiological rationale of the usual classification does not appear to 

 him to work well in detail. Thus although Sporozoa are predomin- 

 antly sluggish, often laden with reserve products, and frequently 

 encysted, (a) they have in many cases flagellate reproductive imits 

 (as in some Coccidians). Again {b) the sporozoites of the malaria 

 organism are for a time amoeboid, and (c) the same organism has 

 microgametes that resemble spermatozoa, (d) Transitory pseudo- 

 podia occur in many Flagellates (e.g. Oikomonas and Dimorpha) 

 and permanent pseudopodia in a few (e.g. Mastigamoeba). (e) Among 

 Radiolarians flagellate zoospores are the rule, and also known to 

 occur in some Foraminifera and Heliozoa. This list of cases, which 

 seems at first sight damaging, if not even contradictory to the theory 

 of three main physiological trends, might be greatly enlarged. But 

 what we suggest is not that the acquirement and predominance 

 of a flagellate, an amoeboid, or an encysted habit excludes the 

 possibility of such passage to another phase, but rather the very 

 opposite. It is what we should expect, from our general theory of 

 the nature of the cell-cycle ; and also as a recapitulation of ancestral 



