EVOLUTION 941 



cytoplasm drift as Plankton in the open waters. Our answer is: 

 all these correlations function both ways, throughout develop- 

 ment and later life. Parasites are sluggish and highly fed: but we 

 often see their active larval forms settling down — which is getting 

 sluggish — and feeding highly — as hungry, as well as in lack of food. 

 And thereafter they settle into their new habits, often very increas- 

 ingly with inactivity of other functions, as the degeneration of their 

 organs shows. Again, in its active mobile state, the nascent cell-form 

 can move swiftly, pursue its food, and so selectively advance in 

 mobility, the amoeboid form creeps, the resting can but absorb, 

 so again selectively. And shell formation, calcareous or siliceous 

 deposition, is reasonably to be associated with less activity, of 

 excretion especially. (Bone cells, etc., are not the active ones.) 

 Also that all these types are doubtless adapted in detail to particular 

 habitats and diets, e.g. Noctiluca with its strong macro-flagellum, 

 Radiolarians with their vacuoles, Foraminifera with their food- 

 capturing network of interlacing filamentous pseudopodia, Gregarines 

 with their semi-permeable cortex, and so on ; but that there is some- 

 thing prior to all this, namely the establishment of physiological 

 types — flagellate, amoeboid, and encysted — which subsequently 

 found their most appropriate habitats and were further differen- 

 tiated in endless adaptive detail. Yet these phase-forms, in our 

 view especially, need not be of single and common origin, but 

 may arise again and again. Thus our interpretation is rather con- 

 firmed than shaken by these criticisms, and we adhere to our 

 view that the grouping of the Protozoa conforms with the three 

 main alternatives of cell-life; and especially when viewed as 

 phases of the cell-cycle : while this again is rationalised physiologi- 

 cally, as outlined at the outset. This thesis might be followed further 

 with illustrations of the same alternatives within the several 

 classes and their orders. 



We thus interpret the well-known Protozoan classes as but so 

 many less or more enduring forms of the cell-cycle, and as becoming 

 specialised on one or other level of functioning, and thus into their 

 usual forms, and so again for most of, if not all, their life; yet 

 often manifesting returns to this, both in individual adaptation 

 to environmental change, or in reproduction, and in individual 

 development. 



The simplest way to interpret all this is no doubt to leave the 

 old classes as monophyletic, distinguishing Flagellates and Ciliates 

 as of early divergence. 



But is not such a classification too simple, even to artificiality: 

 and needing revision accordingly? To choose an extra case: since 

 even in the extreme and encysted passivity of Gregarines the cell- 

 cycle reappears in reproduction, why may not its motile or creeping 

 forms, in favourable circumstances, give rise to their new phyla. 



