OPALINID CILIATE INFUSORIANS — METCALF 613 



seen that the Opahnidae arose in southern, perhaps sub-Antarctic, 

 lands as early as the Triassic period.^^ ^^^^ ^^ ^^lese archaic forms 

 are living in the same regions to-day and are almost unmodified. 

 During at least part of the Triassic period and all the Jurassic, the 

 Cretaceous, and the Tertiary periods, these primitive species {Pro- 

 toopalinae of subgeneric group I) have persisted. The bell toad 

 parasites (Profoopalinae, group II) are practically unmodified after 

 persisting at least since the Jurassic period ; and so on for other genera 

 and species for different lengths of time. The widely evidenced gen- 

 eral principle that evolution is rapid during periods of environmental 

 change and is slow during periods of environmental uniformity receives 

 added support from the Opalinidae. Evolution may proceed under 

 the influence and control of internal factors, but it is likely to be 

 speeded up when environmental pressure (change) and internal 

 responses are operating together, but under such circumstances it is 

 difficult to give proper relative credit to the two sets of factors. 



The stream of protoplasm that was, in the Triassic period, and still 

 is Protoopalina has formed many eddies, species, along its course. At 

 least 48 of these eddies, and probably others not yet discovered, have 

 persisted through many milhons of years until the present time. 

 Doubtless others have disappeared, but the persistence of such 

 species, eddies, when once formed, is remarkable. Some of these 

 eddies, Profoopalinae of subgenus I, arose in the Triassic period; 

 others, subgenus II, probably at the time of the development of the 

 bell toads, which may have been as late as the earliest Cretaceous, 

 but not later, although they were probably of Jurassic origin. Still 

 others, subgenus VIII, were present at a little later period in Asia- 

 Malaysia when Cepedea evolved. The species of subgeneric group VI 

 (parasites of Scaphiopus) did not form until some time in the Ter- 

 tiary, perhaps late in the Tertiary. Through all this immense stretch 

 of time from the Triassic into the Tertiary the stream was forming 

 new eddies and occasionally an eddy divided forming two or more 

 (4 in the case of Protoopalina, subgenus VI), and when formed they 

 tended to persist, or at least many of them did. The successive eddies 

 diverge from one another by slight increments of difference, the 

 species now found forming a remarkably complete series with no 

 great gaps. Speciation in Protoopalina has not been by sportmg, by 

 sudden, extensive mutation, but by changes that have been very 

 gradual, almost every conceivable intergrade between the imperfectly 

 binucleate P. primordialis and the multinucleate P. axonucleata being 

 found. This series shows the changes in line with the trend to multi- 

 plication of nuclei. Especiallyin subgenus VI, very late m this series, 

 the increments of change between species are slight, lor furtlicr 

 illustration of the very minute increments of change, mutation, by 



13 Probably not much, if any, earlier, for tbe Anura are not much oMcr. 



