236 WHALES 



(1951), who investigated the eye muscles of a number of different species, 

 had Httle to add except that the four recti muscles were rather small, while 

 the two obliques were of normal size. The eyeballs of living dolphins were, 

 in fact, found to be very mobile. 



We have now revie\ved the most important characteristics of the Ceta- 

 cean eye, and can pass on to a discussion of the other senses. 



We need not \vaste much time on the sense of smell which is completely 

 lacking in Odontocetes and rudimentary in Mysticetes. Russian zoologists 

 have admittedly found that the Beluga reacts to smoke and other strong 

 odours, but the lack of an olfactory organ would seem to indicate that 

 the stimuli are received by taste receptors or by some other sense. The fact 

 that Cetaceans cannot smell is not so much due to their aquatic life, as 

 to their descent. Smelling, i.e. the perception of chemical stimuli by 

 means of an organ situated in the nose, is perfectly possible in water, so 

 much so that smell forms an extremely important sense in fish which all 

 have highly developed olfactory organs and nerves. Fish can smell the 

 presence of dissolved particles, and their olfactory organ would thus act 

 very much like our own sense of taste. During the evolution of terrestrial 

 animals, however, there occurred a clear separation between these two 

 senses, the organs of taste remaining organs for the perception of particles 

 dissolved in water, while the organs of smell became specialized as 

 perceptors of particles diffused in the air. Since dry and cold air can have 

 an adverse effect on the tender mucous membrane of the organ of smell, 

 it had to be tucked away safely in the back in the nasal cavity where the 

 air can only get to it after being warmed and moistened in the nostrils. 

 Aquatic mammals, however, would choke if water penetrated to the rear 

 of the nasal cavity, and their olfactory receptors would have had to 

 migrate elsewhere. Why they have not is a debatable question, though 

 the answer may well lie in Dollo's 'law of irreversible evolution' (see 

 page 69). By that law, once terrestrial mammals took to water, their 

 olfactory receptors could not travel back to their original position, and, 

 being useless at the back of the nasal cavity, they simply atrophied. In 

 Mysticetes, a vestigial olfactory organ is still present, probably for smelling 

 above the surface, and sea-cows have a fairly well developed organ of 

 smell, no doubt for the same purpose. 



The fact that Mysticetes have retained some sense of smell is inferred 

 primarily from their having small evaginations of the nasal cavity, the 

 ethmoturbinals of other mammals, which are covered with olfactory 

 epithelium. Moreover, their ethmoid, like that of most mammals, has a 

 cribriform plate perforated with small holes, allowing the terminal 

 branches of the olfactory nerve to pass through. The latter, although 

 poorly developed, is present in all Mysticetes and even has a swelling at 



