46 NATURAL SYSTEM OP CLASSIFICATION. 



the other hand, Whales breathe by means of lungs, which require 

 to be filled with air from the atmosphere ; so that these animals 

 are obliged to come occasionally to the surface to breathe. Thus, 

 the function of respiration is conducted on a plan entirely different 

 in these two groups. Again, the heart of the Fish has only two 

 cavities, and the blood does not return to it after passing through 

 the gills, but is immediately distributed to the body : whilst the 

 heart of the Whale has four cavities, and the blood returns to it 

 after passing through the lungs. Hence, the plan of the circula- 

 tion also is entirely different in the two classes, being single in 

 the one, and double in the other. Again, the blood of the Fishes 

 is cold, and that of the Whales is warm ; another character of 

 great importance, in regard to the relative activity of the vital 

 operations in general, in these two classes respectively. Further, 

 Fishes are oviparous, propagating eggs, from which the young 

 come forth in due time, with little or no attention on the part of 

 the parent ; whilst Whales are viviparous, producing their young 

 alive, and nourishing them afterwards by suckling. There are 

 also many other points in the formation of the skeleton, the ner- 

 vous system, the organs of secretion, &c. in which the Whales 

 differ entirely from Fishes, and correspond with Mammals ; and 

 these embrace all the essential particulars of their structure and 

 physiology. The points of resemblance are only adaptive ; being 

 such as enable the Mammal to become an inhabitant of an aqua- 

 tic element. For if we were required to modify a Mammal, in 

 such a manner that it might lead the life of a Fish, we should 

 necessarily alter the form of its body, so that it might be enabled 

 to swim through the water with as little resistance as possible ; 

 and we should also adapt its instruments of propulsion to the 

 new medium in which they are to act, and in which the feet of 

 a terrestrial quadruped would be quite useless to it. 



26. Hence, then, in a natural arrangement, Whales are placed 

 among the Mammals, as agreeing with them in all essential cha- 

 racters; although their external forms, and their mode of life, are 

 so different. To place them with Fishes, on account of their 

 resemblance in external aspect, would be to adopt an artificial 

 principle of classification ; that is, a principle which leads us to 



