460 The Outline of Science 



Water-Shrew has special hairs on the sole and toes of its hind- 

 foot, which are spread out like a comb in swimming, but become 

 appressed when the little creature runs on land. The long tail 

 of the Water-Shrew serves as a rudder; it is somewhat flattened 

 vertically and bears a fringe of long hair on its ventral surface. 

 The adaptations to aquatic life are many: thus there is often a 

 reduction of friction by the disappearance of external ears, as 

 in seal and whale; hair is almost quite gone in cetaceans, though 

 those that remain about the mouth may be very useful in their 

 exquisite tactility ; the absence of hair, which normally serves as a 

 non-conducting robe, is compensated for by the development of 

 a layer of blubber just an exaggeration of the deposit of fat 

 (panniculus adiposus) which is formed under the skin in most 

 mammals (the Common Hare a noteworthy exception) ; the 

 mother whales have an arrangement for giving their baby a huge 

 mouthful of milk at a gulp, for suckling cannot be very easy in 

 the open sea. It is said that the Northern Right Whale may 

 remain under water for an hour and twenty minutes, and in 

 adaptation to this prolonged immersion there is a huge chest 

 cavity, and also a development of wonderful networks (retia 

 mirabilia) of arteries which store pure blood and keep the tissues 

 oxygenated when respiration in the ordinary sense has come to 

 a standstill. According to Lillie, a rorqual may remain eight 

 to twelve hours under water, and it is possible that in this case 

 a sort of skin-respiration (familiar in frogs, for instance) is 

 effected by means of numerous very vascular longitudinal ridges 

 on the underside of the rorqual's throat. Besides the positive fit- 

 nesses, of which some illustrations have been given, there are 

 negative adaptations. Thus, in thoroughly aquatic mammals, 

 such as whales, there can be no smelling, and the olfactory organ 

 is naturally degenerate. For what is useless is rarely conserved. 

 Or, again, the cetaceans, which have their eyes continually washed 

 with water, have no third eyelid which is used in other mammals, 

 except man and monkeys, for cleaning the front of the eye. 



