ON THE ORIGIN OF GENERA. 37 



are strikingly affected, and are chiefly used in guessing at tlie 

 parentage. This is among Cyprinidae so much the case that there 

 is scarcely an example of a hybrid between two species of the same 

 genus brought forward, but often between species of different 

 genera. 



y. Ascertained Cases of Transition. 



This naturally suggests that, in accordance with the theory of 

 acceleration and retardation, a transition can take place in the 

 life history of species. Have we any means of proving this sus- 

 picion ? 



1. The genus Ameiva (Saurians of South America) has been 

 composed of species of moderate size furnished with acutely tri- 

 cuspid teeth. Teius, on the other hand, embraces very large spe- 

 cies with the molars obtusely rounded and of the grinding type. 

 These genera are generally held to be well founded at present. I 

 find, however, that in Ameiva pleii, which is the largest species of 

 the genus, in adults the greater part of the maxillary and mandib- 

 ular teeth lose their cusps, become rounded, then obtuse, and 

 finally like those of Teius. While young, they are true AmeivaB. 

 Strangely enough the A. pleii, from Porto Rico, acquires but 

 three such obtuse teeth when of the size of the other (St. Croix) 

 forms. In youth the teeth of all are as in other Ameivse. Here is 

 a case of transition from one genus to another in the same species. 



2. In the important characters of the possession of branchiae, 

 of maxillary bones, and of ossified vertebrae, the tailed Batrachia 

 presents a series of a rising scale, measured by their successively 

 earlier assumption. Thus Salamandra atra^ produces living 

 young, w^hich have already lost the branchiae ; S. maculosa living 

 young with branchiae ; Plethodon f produces young from eggs 

 which bear branchiae but a short time, and do not use them func- 

 tionally ; Desmogantlms nigra uses them during a very short 

 aquatic life ; D, fusca and other Salamanders maintain them 

 longer ; while Spelerpes preserves them till full length is nearly 

 reached. Finally, species of Amblystoma reproduce while carry- 

 ing branchiae, thus transmitting this feature to their young as an 

 adult character. And it is a very significant fact that Spelerpes, 

 which bears branchiae longest, next to Amblystoma, is associated 



* See Scbreiber's " Isis," 1833, p. 527; Koeliker, "Zeitschr. f. wissensch. Zoolo- 

 gie," ix, p. 464. 



f Baird, " Iconographic Encyclopaedia " ; Wyman, Cope. 



