36 INTRODUCTION. 



temperature ; but fishes are cold-blooded, and their 

 respiration is effected by branchiae, which consist of 

 laminae, suspended on arches, between the head 

 and body, through which the water entering by 

 the mouth is allowed to escape, and so purifies the 

 blood for renewed circulation. On their surface 

 scales almost invariably take the place of the warm 

 and woolly covering : and finally, for it is unneces- 

 sary to carry the distinction farther, the former, as 

 their name implies, move upon the earth with a 

 firm tread, endowed with two limbs belonging to 

 the anterior, and two to the posterior extremity; 

 whilst fish are totally deprived of these, their only 

 representatives being the fins, and they effect their 

 movements through the liquid element mainly by 

 means of their tail striking the water vertically and 

 alternately from right to left. 



These general observations will suffice to enable 

 oar readers to understand distinctly the singular 

 position which is occupied by the Cetacea in the 

 classification of the animal kingdom. While they 

 inhabit the water like fishes, and while in their 

 mode of progression through their common ele- 

 ment and in some of their more obvious external 

 characters, they seem to claim kindred with the 

 other inhabitants of the deep, yet, in every essen- 

 tial respect, they are unequivocally marked as 

 members not of this the fourth and last class, but of 

 the first and most remote class of the vertebratae, viz. 

 the Mammalia. Fish, as we have seen, are produced 

 from spawn, and after the lapse of weeks or months, 



