Chapter Four 



The Senses 



VISUAL SENSE 



(CERTAIN adjustments in the optic equipment of mammals that habitu- 

 ally seek their food beneath the surface of the water is to be expected. 

 The platypus, most of the insectivores and some of the smaller aquatic 

 rodents secure their food largely through the sense of touch. It seems 

 that they must actually keep their eyes closed for a large part of the 

 time which they spend submerged, and the result is the same as is found 

 in fossorial mammals, namely, a tendency toward reduction in the size 

 of the eye. Among Cetacea it is interesting to note that Platanista 

 at least has followed this same course, for it has become virtually blind 

 after having lived for a great length of time in the waters of muddy 

 rivers. 



Other sorts of mammals may have experienced a pressing need to 

 watch for enemies above the water while keeping a maximum amount 

 of head and body hidden from view. The result of this is best exem- 

 plified among mammals by the common hippopotamus, with its dor- 

 sally protruding orbits. The pigmy hippopotamus (Choeropsis), how- 

 ever, does not have markedly protruding orbits but appears more like 

 a young individual of its larger cousin, indicating that the latter is prob- 

 ably derived from a more generalized ancestry not vice versa. The 

 hair seals (Phocidae) show a tendency to acquire dorsal direction of 

 vision to a considerable extent and where this is pronounced there is 

 osteological indication of it in extreme interorbital constriction. The 

 upward pointing of the axis of the eye is to be noted in the sea-lion 

 (Otariidae) and walrus (Odobenidae) also but to a lesser degree. The 

 eye in at least the majority of the Phocidae is relatively larger than in 

 other pinnipeds, possibly because these seals are more in the habit of 

 seeking smaller prey in the subdued light of deep waters, although it 

 must be admitted that the dorsal direction of the eyes would hinder such 

 action. Incidentally I have found scores of half inch shrimps in the 

 stomach of a Phoca hispida, which fare is probably too insignificant to 

 interest an otariid. 



[61] 



