56 TOOTHED WHALES AND THEIR ANCESTRY. 



in place of the two sets of paired fins characterizing a 

 fish, a whale has but a single pair of flippers representing 

 the greatly modified fore limbs of other mammals. 

 The hind limbs have, indeed, been completely lost 

 externally, although more or less imperfect traces of 

 them may still be detected deeply imbedded among the 

 muscles of the body. In the great majority of the^ group 

 the back is furnished with an upright fin, very similar in 

 appearance to the unpaired back-fin of a fish. "Whereas, 

 however, such a back-fin is constantly present in fishes, 

 in cetaceans it may be absent or present in different species 

 of the same genus ; while if we were to cut through such a 

 fin we should find a total absence of the slender spine-like 

 bones characterizing those appendages in a fish ; a similar 

 condition also obtaining in the flukes. In marked contrast 

 to the scaly armour of the majority of modern fishes, the 

 skin of a cetacean is for the most part completely naked ; 

 although the frequent presence, in the young state at 

 least, of a few scattered bristles in the region of the mouth 

 is of itself sufficient to indicate the derivation of these 

 strangely modified creatures from more ordinary mammals. 

 As regards their coloration, we may again, however, note 

 a similarity to most pelagic fishes, in that while the upper 

 parts are generally dark, the lower surface of the body is 

 of a light hue ; this arrangement being, of course, designed 

 to render all these animals as inconspicuous as possible 

 when viewed in the water either from above or from below. 

 Although the flippers show no external indications of 

 toes, and are unprovided with nails, yet their internal 

 skeleton comprises the same elements as occur in the 

 limbs of any ordinary mammal, and is thus quite difl'erent 

 from that of a fish. This structural similarity is, however, 

 to a certain degree obscured by the alteration in the form of 

 the bones, and also by the circumstance that the number 

 of joints in the skeleton of the individual toes is increased 

 beyond the normal. As external ears would be mere 

 useless incumbrances, these appendages are absent ; while 

 the aperture of the ear itself is reduced to an extremely 

 minute size. To prevent the ingress of water during the 

 periods of submergence, the apertures of the nostrils, which 



