THE OPALINID CILIATE INFUSORIANS. 357 



which have persisted for several geologic periods substantially un- 

 modified. 



This persistence of archaic forms is a rather widely spread phe- 

 nomenon among parasites. Having once met the conditions of para- 

 sitism and having undergone the initial inodification to adapt them 

 to the conditions of the new environment, some of their species are 

 rather prone to persist without much further change, living as they 

 do in a secluded, protected, and remarkably uniform habitat. Para- 

 sites may thus present peculiarly dependable evidence in such ques- 

 tions as Ave have discussed in this chapter. 



How long a period has been required for the evolution of the 

 Opalinidae and their four genera? We have seen indication that 

 Protoopalina was present in the Triassic period, and it may have 

 evolved earlier. There are present today in Australia, Papua, 

 tropical Africa, and South America, species which are very closely 

 similar to one another and which probably are in almost the same 

 condition which they had reached during the Triassic. Other groups 

 of species of the Archais genus Protoopalina seem to have arisen in 

 the Jurassic or late Cretaceous (groups 3, 4, 6 ?, 7 ?, and 9), in the 

 middle or later Cretaceous (groups 5, 6 ?, and 7 ?), and in the 

 Tertiary (group 8). The genus ZellerieUa evolved from Proto- 

 opalina apparently during the Miocene. Cepedea evolved from Pro- 

 toopalina probably during the Jurassic or late Cretaceous. The 

 period of origin of some of its groups of species does not seem to be 

 indicated by our data (divisions 2 and 3). The time of origin of 

 other groups is indicated. Division 1, the most archaic division, 

 must, of course, have arisen as early as the Jurassic or early Creta- 

 ceous, and division 2 probably arose during the same period. Divi- 

 sion 6 apparently arose late in the Cretaceous. Division 5 seems 

 to be of Tertiary origin, and the Japanese species, C. fujiensis, 

 may well be of quite late origin, since the time when Japan became 

 isolated from continental Asia. The earliest Opalinae were the 

 Opalinae latae, which doubtless evolved from Cepedea. They ap- 

 parently arose at some time after the middle of the Tertiary and not 

 later than the Pliocene. The Opalinae angustae evolved from the 

 Opalinae latae^ probably during the Pliocene. 



There is another aspect of the matter that is of interest. Secluded 

 and protected in their uniform habitat, the Opalinidae have been 

 but little exposed to the selective action of the struggle for exist- 

 ence. Under these conditions orthogenetic trends have had unusually 

 free play and the resultant speciation is to an unusual degree an 

 expression of the undisturbed outworking of the natural tendencies 

 in the animals themselves. 



Unfortunately for the study of orthogenetic trends, conditions of 

 internal parasitism tend to cause an early simplification in struc- 



