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 
Form. 1 * 
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. III.— i 
