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



they correspond with the Tadpole of the Frog at an advanced period of its 

 metamorphosis ; 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 manner, as to be amply supplied with food, and exposed to a constantly- 

 renewed current of water, but be secluded from light and from the direct in- 

 fluence of the solar heat, they will continue to grow as Tadpoles ; their me- 

 tamorphosis 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 below that to which it is ultimately to belong. This kind of meta- 

 morphosis is by no means confined to them, however ; for the gradual exten- 

 sion of our knowledge of the early history of the different tribes of animals, 

 is constantly bringing to light new facts of the same kind. The Polypes and 

 lower Mollusca, 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 or- 

 gans 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 under- 

 goes a series of progressive changes equally remarkable; the principal differ- 

 ence being, that these changes 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. 



9. General characters of Birds. 



33. 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 effected by the now-extinct Pterodac- 

 tylus, 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 warm-blooded, and in affording as- 

 sistance by their own heat in the development of the ovum. Birds correspond 

 with Mammalia, in possessing a heart with four cavities, and a complete dou- 

 ble circulation ; by which the whole of the blood that has circulated 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 accompa- 

 nied 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 ordinarily meet with among the 

 Mammalia. The temperature of Birds ranges from about 104 to 112. The 

 lowest is in the aquatic species, whose general activity is much less than that 

 of the tribes which spend most of their time in the air; the highest is among 

 those distinguished for the rapidity and energy of their flight, such as the 

 Swallow. 



34. Birds have been denominated, and not inappropriately, the Insects of 

 the Vertebrated series; as in the animals of that class, we find the whole 

 structure peculiarly adapted to motion, not in water, nor upon solid ground, 

 but in the elastic and yielding air. It is impossible to conceive any more 

 beautiful series of adaptations of structure to conditions of existence, than 

 that which is exhibited in the conformation of the Bird, with reference to its 

 intended mode of life. In order to adapt the Vertebrated animal to its aerial 

 residence, its body must be rendered of as low specific gravity as possible. 

 It is further necessary that the surface should be capable of being greatly ex- 

 tended; and this by some kind of appendage that should be extremely light, 



