50 ON THE PLACE OF MAN IN THE SCALE OF BEING. 



the Serpent, is in like manner known to be really allied to the Frog. An 

 acquaintance with the history of these animals confirms such an arrangement, 

 by showing that the Salamander and the Coecilia* undergo a metamorphosis ; 

 breathing by gills, and having the general structure of Fishes, in the early 

 part of their lives. 



43. Besides those animals, however, which attain the condition of perfect 

 Reptiles, this group contains several whose development is arrested, as it were, 

 in an intermediate or transition state ; their adult form presenting a remarka- 

 ble mixture of the characters of the two classes, which they thus connect. 

 This is the case in the Proteus, Siren, and other less known species, which 

 retain their gills through the whole of their lives, whilst their lungs are at the 

 same 'time developed ; so that, as they can respire in either air or water, they 

 are the only true amphibious animals. In their entire organization, they 

 correspond with the Tadpole of the Frog at an advanced period of its metamor- 

 phosis ; and it is a most interesting fact (which has been established by the 

 experiments of Dr. W. F. Edwards) that if Tadpoles be kept in such a man- 

 ner as to be freely supplied with food, and exposed to a constantly-renewed 

 current of water, but be secluded from light and from the direct influence of 

 the solar heat, they will continue to grow as Tadpoles ; their metamorphosis 

 being checked. The metamorphosis of the Batrachia closely corresponds with 

 that of Insects ; the young animal, in each case, at the time of its emersion 

 from the egg, having a resemblance, in all essential particulars, to a class be- 

 low that to which it is ultimately to belong. This kind of metamorphosis is 

 by no means confined to them, however ; for the gradual extension of our 

 knowledge of the early history of different tribes of animals, is constantly 

 bringing to light new facts of the same kind. The Polypes and lower Mollus- 

 ca, for instance, come forth from the egg, and swim about for some time, in a 

 condition which can scarcely be termed animal; for there is not even a mouth 

 leading to a digestive cavity, nor are there any other organs of locomotion than 

 the cilia, the action of which is involuntary. And, in tracing the development 

 of the Human embryo, we shall find that it undergoes a series of progressive 

 changes equally remarkable; the principal difference being, that these changegf 

 are not so arranged in harmony with each other, as to cause the embryo to 

 present, at any one time, the combination of characters which belong to the 

 Fish, Reptile, &c., or to enable it to sustain an independent existence. 



General characters of Birds. 



44. From Reptiles to BIRDS, the transition would seem rather abrupt ; since 

 the latter class is, in almost every respect, the opposite of the former. Never- 

 theless it would seem to have been affected by the now extinct Pterodactylus, 

 which combined in a most remarkable degree the characters of the two groups. 

 Birds are, like Fishes and Reptiles, oviparous Vertebrata; but they differ 

 essentially from both, in being themselves warm-blooded, and in the assistance 

 which they afford by their own heat in the development of the ovum. Birds 

 correspond with Mammalia, in possessing a heart with four cavities, and a 

 complete double circulation ; by which the whole of the blood, that has circu- 

 lated through the body, is exposed to the influence of the air, before being 

 again transmitted to the system. This high amount of oxygenation of the 

 blood is accompanied by great activity and energy of all the organic functions, 

 acuteness of the senses, and rapid and powerful locomotion ; as well as by the 

 evolution of a degree of heat, superior to that which we ordinarly meet with 

 among the Mammalia. The temperature of Birds ranges from about 104 to 



* This fact, in regard to the Ccecilia, has only been recently substantiated. See Annals 

 of Natural History, May 1841. 



