198 THE POPULAR SCIENCE MONTHLY. 



needed, and hence we find these parts in a most rudimentary condition 

 or altogether absent. In the same way the organ of hearing in its 

 essential structure is entirely mammalian, having not only the sacculi 

 and semicircular canals common to all but the lowest vertebrates, but 

 the cochlea, and tympanic cavity with its ossicles and membrane, all, 

 however, buried deep in the solid substance of the head ; while the 

 parts specially belonging to terrestrial mammals, those which collect 

 the vibrations of the sound traveling through air, the pinna and the 

 tube which conveys it to the sentient structures within, are entirely or 

 practically wanting. Of the pinna or external ear there is no trace. 



The organ of smell, when it exists, offers still more remarkable 

 evidence of the origin of the Cetacea. In fishes this organ is specially 

 adapted for the perception of odorous substances permeating the 

 water ; the terminations of the olfactory nerves are spread over a 

 cavity near the front pai't of the nose, to which the fluid in which the 

 animals swim has free access although it is quite unconnected with the 

 respiratory passages. Mammals, on the other hand, smell substances 

 with which the atmosphere they breathe is impregnated ; their olfac- 

 tory nerve is distributed over the more or less complex foldings of the 

 lining of a cavity placed in the head, in immediate relation to the pas- 

 sages through which air is continually driven to and fro on its way to 

 the lungs in respiration, and therefore in a most favorable position for 

 receiving impressions from substances floating in that air. The whale- 

 bone whales have an organ of smell exactly on the mammalian type, 

 but in a rudimentary condition. In the more completely modified 

 Odontocetes the olfactory apparatus, as well as that part of the brain 

 specially related to the function of smell, is entirely wanting, but in 

 both groups there is not the slightest trace of the specially aquatic 

 olfactory organ of fishes. Its complete absence and the vestiges of 

 the aerial organ of land mammals found in the Mystacocetes are the 

 clearest possible indications of the origin of the Cetaceae from air- 

 breathing and air-smelling terrestrial mammalia. With their adapta- 

 tion to an aquatic mode of existence, organs fitted only for smelling 

 in air became useless, and so have dwindled or completely disappeared. 

 Time and circumstances have not permitted the acquisition of any- 

 thing analogous to the special aquatic smelling apparatus of fishes, 

 the result being that whales are practically deprived of whatever 

 advantage this sense may be to other animals. 



All the Cetacea present some traces of teeth, which in structure 

 and mode of development resemble those of mammals, and not those 

 of the lower vertebrated classes, but they are always found in a more 

 or less impei'fect state. 



The meaning and utility of some of the strange modifications in the 

 dentition of whales it is impossible, in the imperfect state of our 

 knowledge of the habits of the Cetacea, to explain, but the fact that 

 in almost every case a more full number of rudimentary teeth is pres- 



