VERTEBRATA. 



cept in very few instances, care for them afterwards ; 

 they are hatched by the heat of the air. In a few cases, 

 however, they are so far advanced before laying, that 

 the act of exclusion breaks the shell, or membrane, and 

 the young are produced alive, as in our commonest 

 Lizard and Viper. The young of the Batrachians 

 (Frogs, &c.) live for some time in the water, and 

 breathe by gills, as Fishes do ; but at a certain pe- 

 riod these are lost, and the matured animal changes 

 its element and mode of respiration. A few of the 

 lowest genera, animals of singular conformation, re- 

 tain their gills through life, and are, consequently, 

 truly amphibious, breathing either air or water at 

 will. 



In these strange forms we see an evident affinity 

 with such Fishes as the Lamprey, while in the Ser- 

 pents there seems a leaning towards the Eels. On 

 the other hand, we have already seen the approach to 

 this Class made by the Penguins and Auks, while 

 the beak of the Turtle strongly aids the resemblance. 

 But the link which most effectually unites these two 

 Classes is most beautifully seen in an extinct animal, 

 called the Pterodactyle.* This must have been the 

 most singular of all known forms, uniting in itself 

 the characters of such different animals as the wildest 

 fancy could hardly have put together. To the neck 

 and head of a Swan it joined a long-pointed beak, 

 beset with sharp teeth ; it had large wings, resem- 

 bling those of the Bats, only that all the fingers but 

 one were free, and of middling size ; this one, how- 



* nrgov, pteron, a wing, and Saxri/Xay, daktylos, a finger. 



