OPALINID CILIATE INFUSORIANS — METCALF 



597 



for one species, solitarius, which spread to the Atlantic coast. Ex- 

 amples of other vigorous, dommant Anura are Bufo, Rana, and the 

 family Leptodactylidae. Rana is a vigorous genus of a vigorous 

 family. Bufo is a vigorous genus in an otherwise seemingly de- 

 cadent family. 



The place of origin of the Hylidae deserves a little further com- 

 ment. The comparative numbers of species in the different regions, 

 two score in Australasia, rather more in North America, and very 

 many in South America seem to indicate origin in South America. 

 On the other hand, the diversity of species north of the Isthmus, 

 forming a number of ''genera," would seem to indicate a long period 

 of evolution and would point to a longer residence in the north than 



-^st 



Figure 152 -Geographic distribution of the Hylidae. The stippled areas indicate the presence or former 

 presence of one species, H. arborea, and closely related forms usually classed as a separate species, but 

 their classification as subspecies of H. arborea would better express the true relationships. The dot in 

 eastern Africa indicates the reputed but very doubtful occurrence in Abyssinia of //. ^racha, not closely 

 related to H. arborea. 



in the South American forests. Two considerations, however, should 

 be held m mind. North of the Isthmus hylids have been exposed 

 to more varied environmental conditions than in the South American 

 forests, especially durmg the clunatic fluctuations of the successive 

 periods of glaciation, and, in the second place, the distmctions 

 between the genera of Hylidae are not worthy of much emphasis 

 from the standpoint of evolution. 



The complete absence of hylids in southern South America, es- 

 pecially when we remember that cold has not prevented their pene- 

 tratmg as far north as Great Slave Lake, is noteworthy and sigiuficant. 

 It is in accord with the fact that in a good many other groups the 



