88 GENERAL EVOLUTION. 



in the same zoological region with a genus (N'ecturus) which dif- 

 fers from its four-toed form (Batrachoseps*) in nothing more 

 than the possession of the osseous and branchial characters of its 

 larva, in a permanent and rej^roducing condition. That this is a 

 genus, to be one day converted into Batrachoseps by an accelera- 

 tion of its metamorphosis, or that has been derived from it by the 

 reverse process, I am much inclined to believe, f In supj^ort of 

 this I quote the following examination into the time of change of 

 the species of Amblystoma from my essay on that genus : J 



*^ The great difference between the different species, and be- 

 tween individual species in this respect, may be illustrated by the 

 following comparison between the size of the animals at the time 

 of losing the branchiae, so far as known, and that to which they 

 ultimately attain. 



Species. Size at loss of branchi^. 



In. Lines. 



A. jeffersonianum, 1 S'YS 



A. punctatum, 1 10 



A. conspersum, 1 10"5 



A. opacum, 2 2 



A. texense, 2 1 



A. microstomum, 2 3'5 



A. talpoideum, 3 (perhaps too large). 



A. paroticum, 3 7*5 (not smallest). 



.... { S 1 to 



A. tigrinum, j ^ ^ 



A ^' i 3 9-5 to 8 9 



A. mavortium, ■] s 



A. mexicanum, ? branchias persistent. 8 " 



The last species, though not uncommon in collections, is not 

 known to pass through its metamorphoses in its native country, 

 but reproduces as a larva, and is therefore tj^DC of the genus Sire- 

 don of Wagler, Cuvier, Owen, and others. The larva of A. ?na- 

 vortium in like manner reproduces, but their offspring have in 

 the Jardin des Plantes and at Yale College undergone an early 

 metamorphosis.* 



* See Cope, *' Jour. Ac. Nat. Sci.," Phila., 1866. 



f Necturus differs from all true Salamanders in the possession of the os interca- 

 lare of the skull; the parallelism is therefore inexact. (Note, 1886.) 

 X " Proceed. Academy," Phila., 1867. 



* Through the kindness of Prof. Dumeril I have received both larvae and 

 adult of the species here noted, and observed by him. The larva is, as he states, 

 Siredon lichenoides of Baird, while the adult is his Amhlyntoma mavortium^ not A. 

 tigriyium { = luridum), as also supposed by Dumeril. (Note, 1886. I now believe 

 these forms to belong to one species.) 



