THE OPALINID CTLIATE INFUSOKIANS. 25 



divergence have apparently been internal, divergent adaptation to ex- 

 ternal surroundings having had little, if any, part in the process. 

 We seem to have, then, in the members of this family a most instruc- 

 tive illustration of divergent evolution due to the inherent internal 

 tendencies in the original stock, and to such further internal ten- 

 dencies as developed during the progressive history of the group, with 

 little control from outside influences. The environmental conditions 

 have been remarkably uniform and constant. The variable factors 

 in the evolution have been internal. From this viewpoint the specia- 

 tion becomes particularly interesting. It will also be seen to have 

 great interest in connection with questions of the origin of the Ciliata. 

 It might be claimed that the diversity between the different spe- 

 cies of Opalinidae arose in response to diverse conditions in the 

 chemical environment in different species of hosts, but this suggestion 

 is rendered improbable by the conditions among the Opalinae angus- 

 tae^ a group which arose apparently among the Hjdidae (a South 

 American family) when these came into North America during the 

 Pliocene period (see section 7) and there met the Ranas and adopted 

 their broad Opalinae. In the Hylidae the broad Opalinae became 

 narrow, but, as the narrow Opalinae spread to other species and 

 genera of Hylidae and to species and genera of other families, they 

 have been slow to change their character. The forms which I have 

 had to class as Opalina ohtrigonoidea^ a new species, are found in 

 18 species of hosts (Hylidae 6, Bufonidae 5, Pelobotidae 1, Gastro- 

 phrynidae 1, Ranidae 6), yet their diverse hosts have not produced 

 diversities in the parasites sufficient to be recognized as specific. 

 This does not support the idea that the diversities in the species of 

 Opalinidae are due largely to diversities in their chemical environ- 

 ment in their hosts. Factors within the Opalinidae themselves, 

 rather than environmental conditions, seem to have been chiefly con- 

 trolling in their speciation. 



3. A DESCRIPTION OF THE KNOWN SPECIES AND OF THE GENERA 

 IN THE FAMILY OPALINIDAE. 



Twenty-five species of Opalinidae have been described, of which 

 two are doubtful and a third is in this paper demoted to be merely 

 a forma of another species. This paper adds 120 species (of which 

 18 are somewhat doubtful), 20 subspecies (of which 6 are doubtful), 

 and 10 formae. Taxonomic studies in this group are difficult because 

 of several features : 



1. The several species of Opalinids for the most part seem to show 



racial diversities which are much more marked than, say, those of 



Paramecium^ and the extreme forms of different "species" overlap. 



Demarcation of species in some instances seems well nigh impossible. 



83103—23 3 



