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 Otariide 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,! 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”? to the Porpoise 
contains a larger element of truth than the speculations of many 
accomplished zoologists of modern times. The fact that Platanisia, 
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 Marsowin. ‘‘ Porpoise” is said 
to be derived from ‘‘ Porc-poisson.”’ 
