THE SENSES 



the hippopotamus has had need to watch for foes approaching hori- 

 zontally along the river banks. At any rate it seems likely that only 

 those aquatic mammals which have experienced a definite need for 

 eyes directed more dorsally will have acquired this characteristic, and 

 one would not expect it to develop to as high a degree in the great 

 majority of cases as it now occurs in the anomalous hippopotamus. 

 For one thing, unless the need for it was extraordinarily strong, by the 

 time that dorsal vision had become moderately efficient it is probable 

 that aquatic specialization in other respects would usually be so far 

 advanced that the animal would have but little reason to fear enemies 

 approaching above the surface of the water. 



As soon as the last mentioned stage in aquatic specialization has been 

 reached the occular stimulus would automatically change, for the chief 

 need, and probably the only one, would then be for the discernment 

 of submarine food and enemies. In the case of a mammal whose eyes 

 had already turned upward to a greater or lesser amount, there would 

 then be a secondary migration downward of the direction of vision, 

 so as better to detect food which it was approaching. For all we know 

 the progenitors of the whales may have passed through just this visual 

 cycle, in which case there would have been left some complication in 

 skull development which one cannot hope to decipher by means of the 

 fossils now available. 



Horizontal direction of vision may be either forward or lateral, or 

 even backward. If the former then it must presumably be binocular, 

 and if lateral, monocular. We have no means of knowing which of 

 these sorts of sight might prove most useful to an aquatic mammal but 

 it seems likely that binocular vision would never be developed by any 

 reasonably active mammal of high aquatic specialization. This would 

 entail a forward direction of the orbits where the eyes would receive 

 the full force of water pressure as the animal progressed — a position 

 which obviously would prove of considerable, and perhaps critical, 

 detriment. As a matter of fact the eyes of the Cetacea are directed at 

 practically a perfect right angle to the body axis, where they receive 

 the minimum of friction and irritative interference by the water. 



Eyelids that are at least partly functional are retained by aquatic 

 mammals. In pinnipeds, serenians (apparently) and mysticetes these 

 are largely comparable to what are found in terrestrial mammals, with 

 lids moderately wrinkled, indicating that they may easily be closed 

 and held in that position without strain as long as the animal wishes. 

 This statement should perhaps be qualified as regards the whalebone 



[63] 



