GENERAL CHARACTERS 233 



established. The steps by which a land mammal may have been 

 modified into a purely aquatic one are indicated by the stages 

 which still survive among the Carnivora in the Oturiidw and in 

 the true Seals. A further change in the same direction would pro- 

 duce an animal somewhat resembling a Dolphin ; and it has been 

 thought that this may have been the route by which the Cetacean 

 form has been developed. There are, however, great difficulties in 

 the way of this view. Thus if the hind limbs had ever been 

 developed into the very efficient aquatic propelling organs they 

 present in the Seals, it is not easy to imagine how they could have 

 become completely atrophied and their function transferred to the 

 tail. So that from this point of view it is more likely that Whales 

 were derived from animals with long tails, which were used in 

 swimming, eventually with such effect that the hind limbs became 

 no longer necessary. The powerful tail, with its lateral cutaneous 

 flanges, of an American species of Otter (Lutra brasiliensis) may give 

 an idea of this member in the primitive Cetaceans. But the struc- 

 ture of the Cetacea is, in so many essential characters, so unlike 

 that of the Carnivora that the probabilities are against these orders 

 being nearly related. Even in the skull of the Zeuglodon, which 

 has been cited as presenting a great resemblance to that of a Seal, 

 quite as many likenesses may be traced to one of the primitive Pig- 

 like Ungulates (except in the purely adaptive character of the form 

 of the teeth), while the elongated larynx, 1 complex stomach, simple 

 liver, reproductive organs both male and female, and foetal mem- 

 branes of the existing Cetacea are far more like those of that group 

 than of the Carnivora. Indeed it appears probable that the old 

 popular idea which affixed the name of " Sea-Hog " 2 to the Porpoise 

 contains a larger element of truth than the speculations of many 

 accomplished zoologists of modern times. The fact that Platanista, 

 which, as mentioned above, appears to retain more of the primitive 

 characteristics of the group than any other existing form, and also 

 the somewhat related Inia from South America, are both at the 

 present day exclusively fluviatile, may point to the fresh-water origin 

 of the whole group, in which case their otherwise rather inexplic- 

 able absence from the seas of the Cretaceous period would be 

 accounted for. 



On the other hand, it should be observed that the teeth of the 

 Zeuglodonts approximate more to a carnivorous than to an ungulate 

 type. It is scarcely necessary to allude to the hypothesis started 

 by some Continental writers to the effect that the Whales are the 

 most primitive type of mammals with which we are acquainted, 



1 There is much resemblance in the larynx of the Hippopotamus, but none 

 in that of the Seal, to the same organ in the Cetacea. 



2 German Meerschwein, whence the French Mar sou in. "Porpoise" is said 

 to be derived from " Parc-j)oisson." 



