EXTERNAL FEATURES 



seals or phocids and most of the eared seals or otariids, in which the 

 pelage is of course hair only, and the fur seals, famous for the softness 

 and denseness of their undercoat. Nothing, of course, is known regard- 

 ing the pelage of the pinniped ancestors and it is useless to speculate. 

 Undoubtedly the progenitors of the walrus had a coat of some sort, 

 which was largely lost for reasons unknown. That the earless seals are 

 derived from an ancestor with a more luxuriant and softer pelage is in- 

 dicated by the fact that at birth some phocids are covered with an ex- 

 cessively long, woolly coat which in some is shed at the age of about 

 one month (and until this time they cannot swim), while in other gen- 

 era this coat is shed in utero. And this is an important reason for con- 

 sidering the phocids to be more primitive phylogenetically than the 

 otariids. Probably contributory to these three conditions of pelage in 

 the Pinnipedia are the facts that the walrus is reputed normally to 

 have the thickest coat of fat and so has less need for hair ; but I cannot 

 see that the need of the fur seal for a warm pelage could be greater than 

 that of the Steller sea-lion, and it is probable that there is an additional, 

 obscure reason for the presence in the former animal of this type of coat. 



The only aquatic mammals with practically naked and rugose hides 

 are the walrus and the sirenians. The former was probably derived 

 from a hairy ancestor and the reason for its present naked condition 

 is obscure. Embryos and young of the latter are said to be more hairy 

 than the adult, which would indicate that the ancestral form was also 

 more hairy. But the sirenians may never have had a thick coat, for this 

 order is commonly believed to have been of the same stock as the 

 proboscidians, and elephants have been prone to a hairless condition. 

 And the latter theory may well account for the rugosity of the hide of 

 these animals. Feeding upon inactive prey and having no great need of 

 speed, a rugose hide and its consequent retardation of speedy passage 

 through the water would doubtless prove of slight detriment. But 

 theoretically no aquatic mammal should attain to high speed and at the 

 same time retain a rugose hide. 



The hippopotamus might well be expected to have a rugose hide but 

 on the contrary it is remarkably smooth for an animal of this size, and 

 its present hairlessness, in a slow mammal that spends much of its time 

 on shore, would indicate a hairless ancestor. Its hide appears to be 

 rather tender in spite of its great thickness, and it is not surprising that 

 some integumental provision has been made for the alternation of a 

 whole day spent in the water followed by long terrestrial excursions at 

 night in a hot and at times a dry climate. The glands of the skin ex- 



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