MOUTH AND NOSE 



been in operation, and the capabilities of an animal for responding to it. 

 Thus the whales are relatively speedy, compared to swimming land 

 mammals ; most of them cover much territory ; and we can well see that 

 they would be greatly handicapped if they could not renew their air 

 supply at full speed in a choppy sea, but had to slow down and twist 

 their nostrils above the surface at every breath. Hence the nostrils would 

 be expected eventually to occupy the position upon the top of the head 

 that they now do. Sea-lions and otters might be expected ultimately to 

 follow the same development. On the contrary an animal like a sirenian 

 or hippopotamus which does not seek active prey but only pokes the nose 

 cautiously above the surface, usually while stationary and in a sheltered 

 lagoon or river, might never attain a like nasal development. 



Nothing much in this respect can be told about the platypus. The 

 nostrils are dorsally located upon the "bill" but we have no means of 

 judging the condition in its terrestrial ancestor. No migration of the 

 nostrils is apparent in any of the insectivores, and in few rodents. Thus 

 the nostrils of the coypu, capybara, beaver and muskrat are considered to 

 be located slightly more dorsal than usual, but the alteration is really very 

 slight. In the hippopotamus the nostrals are definitely upon the dorsal 

 side of the snout, so that when the remainder of the body is submerged 

 the animal may breathe with only the nostrils, eyes and ears exposed; 

 or when thoroughly frightened, the nostrils alone may be quietly thrust 

 forth. If the latter were situated farther back on the head this could not 

 be accomplished without showing the eyes also, and this may militate 

 against any farther migration caudad of the nares. The same may be 

 said of the Sirenia. The nostrils might conceivably fuse to form a 

 single blowhole and this might protrude somewhat so that breathing 

 could be accomplished with the very minimum of exposure, but unless 

 there was a radical change in habits there could be little stimulus for a 

 migration of the nostrils to the top of the head. At present they are 

 not even as dorsally situated as in the hippopotamus, for the animal 

 evidently does not make much use of its eyes for peering above the 

 surface, and all it need do is extrude the muzzle, with the axis of the 

 head at an angle (not parallel) with the surface. 



In a different class are otters, pinnipeds and whales. These either 

 pursue active prey, travel at speed from place to place, or both: The 

 otter is not as yet sufficiently modified for more than a slight change in 

 position of the nostrils to be apparent. And unless there is a great in- 

 crease in size throughout future geologic time the stimulus for posterior 

 migration of the nares will doubtless be very slight. This animal can 



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