ON VARIATION. 71 



III. Teeth in both jaws. 

 a. Sacral diapophyses not dilated. 



Amphignathodontidae \ Ceratobatrachida^. 



Hemiphractidae j 



In strict reference to the structure of the hind feet 

 the following parallels may be drawn : 



FlRMISTERNIA. RANID^. ARCIFERA. 



External metatarsal free : 



Aquatic. Rana. Pseudis. 



Subfossorial. Hoplobatrachus. Mixophyes. 

 External metatarsal attached : 

 Feet webbed- 

 Burrowing. Pyxicephalus. Ceratophrys. 



Arboreal (vom. teeth). Leptopelis. Hyla. 



Arboreal (no v. teeth). Hyperolius. Hylella. 



Aquatic. Heteroglossa. Acris. 

 Feet not webbed 



Terrestrial. Cassina. Cystignathus. 



Terrestrial, spurred. Hemimantis. Paludicola. 



Parallel series like those of the Arcifera and Fir- 

 misternia I have termed "homologous," and the cor- 

 responding terms of such series I have called " hete- 

 rologous." 1 Such corresponding phylogenetic series 

 are homologous to each other, while their terms or 

 genera are heterologous in their relation to correspond- 

 ing terms of other phyla. In such cases the genera or 

 terms of a series owe their resemblances to each other 

 to inheritance ; but they owe their resemblances to 

 their corresponding or heterologous genera, to identi- 

 cal evolutionary influences. Subsequently to my pro- 

 posal to use the above terms, Prof. E. R. Lankester 

 proposed the word "homogenous" to express what is 

 conveyed by my term homologous, and "homoplastic" 

 to express the sense of heterologous. For the two con- 



1" Origin of Genera," Proceedings Philadelphia Academy, 1868, p. 281. 



