WHALES, PAST AND PRESENT. 197 



are absolutely isolated, and little satisfactory reason has ever been 

 given for deriving them from any one of the existing divisions of 

 the class more than from any other. The question has indeed 

 often been mooted whether they have been derived from land mam- 

 mals at all, or whether they may not be the survivors of a primitive 

 aquatic form which was the ancestor not only of the whales, but of all 

 the other members of the class. The materials for I will not say 

 solving but for throwing some light upon this problem, must be 

 sought for in two regions in the structure of the existing members 

 of the order, and in its past 

 history, as revealed by the _, Vv-~-- -7^ ._.__^ ^ > 



discovery of fossil remains. ^^^Ws -- ~ - - 



In the present state of science ^^(Kz: --------- fi^. ~ 



it is chiefly on the former ~^^ s ^^^^^^^^^s^ 



that we have to rely. ofc& 



One of the most obvious ~~-^.~- _' -*^^P^ : 



external characteristics by -7*%- 



which the mammalia arc dis- \ :: ^0^^) ^ < ^5Smi 



tino-uished from other classes - ., *a^%-=-T*w ==-: 



of vertebrates is the more or Hit : J W 



less complete clothing of the ^^^ff^^^^^ ^^^g " 



surface by hair. The Cetacea 



, i , . Fig. 2. Common Porpoise. 



alone appear to be exceptions 



to this generalization. Their smooth, glistening exterior is, in the 

 greater number of species, at all events in adult life, absolutely bare, 

 though the want of a hairy covering is compensated for functionally 

 by peculiar modifications of the structure of the skin itself, the epider- 

 mis being greatly thickened, and a remarkable layer of dense fat closely 

 incorporated with the tissue of the derm or true skin ; modifications 

 admirably adapted for retaining the warmth of the body, without any 

 roughness of surface which might occasion friction and so interfere 

 with perfect facility of gliding through the water. Close examination, 

 however, shows that the mammalian character of hairiness is not en- 

 tirely wanting in the Cetacea, although it is reduced to a most rudi- 

 mentary and apparently functionless condition. 



In the organs of the senses the Cetacea exhibit some remarkable 

 adaptive modifications of structures essentially formed on the mamma- 

 lian type, and not on that characteristic of the truly aquatic verte- 

 brates, the fishes. 



The modifications of the organs of sight do not so much affect the 

 eyeball as the accessory apparatus. To an animal whose surface is 

 always bathed with fluid, the complex arrangement which mammals 

 generally possess for keeping the surface of the transparent cornea 

 moist and protected, the movable lids, the nictitating membrane, the 

 lachrymal gland, and the arrangements for collecting and removing 

 the superfluous tears when they have served their function can not be 



