I 



THE BATEACHIA OF NORTH AMERICA. 249 



The families of Arcifera with opisthocoelous vertebrae are omitted 

 from the table as having no counterpart among the Firmisternia. 



These two series are what I have called " homologous groups," and 

 the corresponding genera " heterologous terms. 1 have also supposed 

 that one such series may have been derived from the other, in evolu- 

 tion, by a change in the one character which distinguishes the two 

 series. In the case of some homologous series it is not unlikely that 

 this may have taken place, but it is necessary, in order to be sure that 

 such has been the process of evolution, to distinguish between two dif- 

 ferent kinds of homologous groups. In one kind the parallel charac- 

 ters of the one group may have been derived from those of the other by 

 descent, according to the principle called by Lankester " homogenj-." 

 lu the other case, like modifications of structure have arisen in differ- 

 ent series of animals as a result of the operation of similar energies, as 

 that of the animal and its environment. This is the princiijle of " homo- 

 plassy." To the latter kind belong the imitations found to exist be- 

 tween the placental and didelphian mammalia. The relation between 

 the arciferous and firmisternial Anura may be one of homogen3^ We 

 may then i^arallelize the families which may exhibit true cases of de- 

 scent as follows : 



Firmisternia. Arcifera. 



Phryuiscidse. Bufonidse. 



Dendrobatidse. Deudrophryniscidie. 



Dyscophidse. Pelodytidse. 



RauidtB. CystigiiathidsB. 



CeratobatracliidsB. Hemiphractidai. 



It is, however, probable that the Pelodytidai is the generalized form 

 from which most of the arciferous families have been deriv^ed ; audit was 

 itself probably a descendant of the families with opisthocoelous verte- 

 brae, as already indicated. 



The Eauidfe embraces many genera which imitate in details many 

 genera of Arcifera. The metropolis of the former, as of the LacertUia 

 acrodonta, is the regio Palcvotropica, while the latter have but few 

 representatives out of the R. R. Neotropica and Australis, where but one 

 or two species of the former occur. In both we can trace a series in 

 which the outer metatarsal is gradually liberated from the penultimate, 

 to afford greater extension for the web in the most aquatic types, and 

 among those where these bones are bound, from webless to webbed 

 types. In both we have burrowing and arboreal genera. 



