218 GENERAL EVOLUTION. 



and last stage, the tympanum is also inclosed behind by bone. 

 Now all of these types are not found in all of the families of the 

 Anura, but the greater number of them are. Six principal fami- 

 lies, four of which belong to the Arcifera, are named in the dia- 

 gram below, and three or four others might have been added. I 

 do not give the names of the genera which are defined as above 

 described, referring to the explanation of the cuts for them, but 

 indicate them by the numbers attached in the plate, which corres- 

 pond to those of the definitions above given. A zero mark signi- 

 fies the absence or non-discovery of a generic type. 



It is evident, from what has preceded, that a perfecting of the 

 shoulder-girdle in any of the species of the Bufoniform and Ar- 

 ciferous columns would place it in the series of Eaniformia. An 

 accession of teeth in a species of the division Bufonifor?ma would 

 make it one of the Arcifera j while a small amount of change in 

 the ossification of the bones of the skull would transfer a species 

 from one to another of the generic stations represented by the 

 numbers of the columns from one to seven. 



There are few groups where this law of parallelism is so readily 

 observed among contemjoorary types as the Batracliia, but it is 

 none the less universal. The kind of parallelism usually observed 

 is that in which there is only a partial resemblance between adults 

 of certain animals and the young of others. This has been termed 

 *^ inexact parallelism," and the relation is presented by forms not 

 very nearly phylogenetically related. The more remote the phylo- 

 genetic lines of two types, the more *"' inexact " will their parallel- 

 ism be. It was once a question whether any parallelism can be 

 traced between the members of the five or six primary divisions of 

 animals, and in my essay on the *^ Origin of Genera," I was com- 

 pelled to state that there was then ^'no evidence of the community 



