272 PROCEEDINGS OF THE ACADEMY OF 



In the preceding diagrams each horizontal column represents the life history 

 of the individuals of each genus. Tlie line of dots, stars, etc., represents 

 the same developmental stage of each, as it appears earlier or later in the life 

 of the individuals. The point of crossing the breeding period is that at 

 which the character is rendered permanent. When the change falls on this 

 period the character is not generic, as in Ixalus, Tab. II. The period of losing 

 the tail, like that of breeding, is represented as occurring at nearly the same 

 time in the history of every genus, as it is generally seasonal. Yet this is not 

 always so, and like the other characters has most likely had its period of 

 shifting. Compare diiference of time of development, for instance, of the 

 frontal and prefrontal bones in Tabs. I. and IV. The comparison of the adult 

 stages of the less developed genera, at the tops of the columns, with the 

 larval conditions of those more fully developed, may be traced in the absence 

 of characters which appear in the latter. I have convinced myself of the 

 accuracy of the above relations by the examination of many skeletons and 

 wet pref)arations of adults and larva?. 



The tables* are representations of nature, and not ideal sketches. It is to 

 be noted as remarkable, that the advance throughout so many diverse groups 

 is in the same direction, viz., to complete or excessive ossification of the cra- 

 nium ; and this identity of progress might be readily shown by adding other 

 characters, were it not thai? the tables would become too complex for con- 

 venience. 



Has any such transition from, genus to genus ever been seen to occur? 



It must of course take place during the life of the individuals of a species, 

 and probably at different times during the lives of different individuals, de- 

 pendent on their relative vigor. In our view, ordinary metamorphosis is such 

 a change, and we have stated its bearing in this form, that '' every character 

 distinguishing suborders, families and genera is to be found among the indi- 

 viduals of some species, living or extinct, to mark new varieties or stages of 

 growth." 



cL. The developmental relation of generic to specrfic characters. 



For the relation of the law of retardation and acceleration to specific char- 

 acters we will look to development again. While the young of Trachycepha- 

 lus are successively different genera, they preserve most of their specific char- 

 acters so as not to be mistaken. Agassiz says of the development of the 

 North American turtles, f "I do not know a turtle which does not exhibit 

 marked specific peculiarities long before its generic characters are fully devel- 

 oped." The same thing can be said of the characters of our salamanders, 

 whose specific marks appear before their generic, or even family characters. 

 I suspect that this will be found to be a universal law. 



It also follows, if a developmental process, as proposed, has existed, 

 that at times the change of generic type has taken place more rapidly than that of 



* Notes on the tables. — I. I characterized a genus Zaphrissa (Journ. A. N. S., 1866) from 

 the Braunkohle (miocene) of Prus.sia, as different from Latonia, on the ground of the 

 presence of a fontanelle in the exostosed frontoparietal bone.s. This combination of 

 characters is very improbable, and appeared so at the time ; but the appearance of the 

 specimen is quite clear in this respect. I think, however, it must be the result of injury, 

 and that the roof has tieen partially carried away. 



Tab. II. Polypedates is here restricted to P. maculatus and P. quadrilineatus. The 

 other species are referred to Rhacophorus, which has not hitherto rested on any proper 

 basis; the asserted character — the palmation of the hands- being one quite graduated 

 from species to species among Hylse. Chiromantis, Peters, is referred to the same, as its 

 character is not strongly marked and is visible in other species. For similar reasons 

 Leptomontis is referred to Ixalus. 



Tab. IV. In each of series II and III two series are mingled for the sake of comparing 

 the structures of the prefontal bones. Thus Heteroglossa, Staurois, Hylorana and Try- 

 pheropsis are one series, and Hyperolius and Hylambates members of another. 



t Contrib. N. Hist. United States, I., p. 391. Note. 



[Oct. 



