APPLICATION OF NATURAL SYSTEM. 79 



some of these characters may be termed natural, because they 

 have an obvious connection with the real or essential differences 

 that distinguish the two groups ; whilst others are considered as 

 artificial, because, though extremely convenient to the Zoologist, 

 they do not of themselves tell him anything respecting those dif- 

 ferences. Thus, the popular error, formerly adverted to, of con- 

 founding Whales with Fishes, may be avoided by attending to 

 either of two very easily- observed characters the first natural, 

 the second artificial. In the Whales, and other Mammals that 

 inhabit the water, the tail is flattened horizontally, and its stroke 

 is vertical, or up and down ; but in Fishes, the tail is invariably 

 flattened vertically, and its stroke is horizontal, or from side to 

 side. This is a natural character, because it is closely connected 

 with the mode in which one of those functions is performed, the 

 plan of which is entirely different in the two groups : the Whales, 

 which breathe air by lungs, require to come frequently to the 

 surface in order to take it in ; and the direction in which their tail 

 strikes the water, enables them to rise or fall in it with great faci- 

 lity: but Fish, which breathe by means of the passage of water 

 over their gills, do not require such a power, and the movement'of 

 their tail is chiefly destined to urge them rapidly through the 

 water. Consequently, we could never expect to meet with a 

 Whale having a tail vertically flattened, nor with a Fish having 

 a horizontally expanded tail ; and this character, therefore, would 

 suffice to distinguish the animals of the two groups, without the 

 aid of any other. But the inspection of a small part of its skin 

 would of itself suffice to determine, whether a given animal were 

 one of the Whale tribe, or a true Fish ; for the skin of the Fish 

 is covered with scales, whilst that of the Whales is destitute of 

 any such protection, and has only a few hairs scattered over it. 

 Now, this must be regarded as an artificial character ; since we 

 cannot discover what essential connection there is, between the 

 internal conformation of the Fish and its covering of scales, or 

 between the arrangement of the internal organs of the Whale, 

 and the scanty covering of hair with which its skin is furnished. 

 Moreover, although no Fish possess hairs, yet some Mammals 

 aie partially covered with a kind of scales; which shows that 



