THE ROYAL NATURAL HISTORY. 



MAMMALS. 



CHAPTER XXIX. 

 Cetaceans, — Order Cetacea. 



Under the general title of Cetaceans may be included the whole of those mammals 

 commonly known as whales, porpoises, and dolphins, which differ from all hitherto 

 described in their assumption of a fish-like form, and their complete adaptation to a 

 purely aquatic mode of life. Indeed, so like are Cetaceans in their general outward 

 appearance to fishes, that they are commonly regarded as belonging to that class. 

 In all essential features of their organisation they are, however, true mammals, 

 breathing atmospheric air by means of lungs, having warm blood, a four-chambered 

 heart, the skull articulating with the first joint of the backbone by means of two 

 condyles, and the cavity of the body divided into two chambers by a midriff; while 

 they produce living young, which are nourished by milk drawn from the bodies 

 of their mothers. 



The assumption of a fish-like form by the Cetaceans is one of the 

 best-marked examples of what are known as adaptive characters, 

 which are merely produced in order to suit the animals in which they exist to their 

 vol. in. — I 



Form. 



