464 THE PROTOZOA 



that the ancestor was Amoeba proteus or Cercomonas crassicauda, 

 but only that it was a form such that, if it existed at the present 

 day, it would be referred by its characters to the genus Amoeba or 

 Cercomonas, as the case might be. 



The data for drawing phylogenetic conclusions in Protozoa con- 

 sist entirely of comparisons between the structure and life-history 

 of the various existing forms. Palaeontology gives no assistance, 

 since only skeletons are preserved as fossils. All that can be 

 learned from the geological record is that the differentiation of the 

 main groups must have taken place at an immeasurably remote 

 period of the earth's history, since skeletons of Foraminifera and 

 Radiolaria groups of which the structure and life-history indicate 

 a long pedigree are found in the earliest fossiliferous strata. It is 

 little wonder, therefore, that the phylogeny of the Protozoa is a 

 subject on which the most opposite opinions are held, as is apparent 

 from the classificatory systems cited above. There can be no 

 finality in a phylogenetic theory, nor, consequently, in any scheme 

 of classification put forward. Both the one and the other express 

 merely the state of current knowledge, and may be expected to 

 undergo modification as knowledge advances. 



It is impossible to discuss here at length the phylogeny and 

 classification of the Protozoa, and only a few guiding principles 

 can be put forward. From a general survey of the phylum, it may 

 be claimed first of all that the Protozoa constitute a compact group 

 with definite characters, not a mere receptacle into which can be 

 put anything and everything of microscopic dimensions which is 

 not a bacterium, a fungus, or a parasitic worm, as some writers 

 seem to think. Common to all Protozoa in at least the principal 

 stages of the life-cycle is the differentiation of the body into distinct 

 nucleus and cytoplasm that is to say, the possession of that type 

 of organization to which I have proposed to restrict the application 

 of the term cell. Doubtless there are, or have been, transitions 

 from this type to the simpler grade of organization characteristic 

 of the bacteria and allied organisms, but such transitions must be 

 sought for outside the phylum Protozoa. 



The essential unity and homogeneity underlying the innumerable 

 differentiations of form and structure in the Protozoa may be taken 

 to mean that the phylum as a whole is descended from a common 

 ancestral form, and the first problem is, then, to attempt to form 

 some notion of what the ancestor was like. In dealing with the 

 more specialized forms, such as those constituting the Infusoria or 

 the two principal subdivisions of the Sporozoa, it has been pointed 

 out that each group appears to be derived either from flagellate or 

 sarcodine ancestors. In reviewing the Mastigophora and Sarcodina, 

 it was further pointed out that, greatly as the typical representa- 



