230 CETACEA 
reptiles, birds, and mammals, the Whale resembles the last-named 
and differs from the fish. It is as essentially a mammal as a Cow 
or a Horse, and simply resembles a fish externally because it is 
adapted to inhabit the same element; but it is no more on that 
account a fish than is a bat, because adapted to pass a great part of 
its existence on the wing in the air, nearly related to a bird. The 
whole structure of a whale is a most instructive instance of a type 
of organisation which is common to and characteristic of the class 
Mammalia, but specially modified or adapted to a peculiar mode of 
life. We see in every part the result of two great principles acting 
and reacting upon each other—on the one hand, adherence to type, 
or rather to fundamental inherited structural conditions, and, on 
the other, adaptation to the peculiar circumstances under which it 
lives, and to which in all probability it has become gradually more 
and more fitted. The external fish-like form is perfectly suited for 
swimming through the water; the tail, however, is not placed 
vertically as in fishes, but horizontally, a position which accords 
better with the constant necessity for rising to the surface for the 
purpose of breathing. The hairy covering characteristic of all 
mammals, which if present might interfere with rapidity of move- 
ment through the water, is reduced to the merest rudiments—a 
few short bristles about the chin or upper lip—which are often 
only present in very young animals; and the function of keeping 
the body warm is supplied by the “blubber.” The fore-limbs, 
though functionally reduced to mere paddles, with no power of 
motion except at the shoulder-joint, have beneath their smooth and 
continuous external covering all the bones, joints, and even most of 
the muscles, nerves, and arteries of the human arm and hand; and 
the rudiments of hind legs found buried deep in the interior of the 
animal apparently subserve no useful purpose, but point an in- 
structive lesson to those who are able to read it. 
As before said, the Cetacea form a perfectly well-defined group, 
sharply separated from all other mammals, and with no outlying or 
doubtful forms at present known. Among the existing members 
of the order, there are two very distinct types, the Toothed Whales 
or Odontoceti and the Baleen Whales or Mystacoceti, which present 
as many marked distinguishing structural characters as are found 
between many other divisions of the Mammalia which are reckoned 
as orders. The extinct Zewglodon, so far as its characters are known, 
does not fall into either of these groups, but is in some respects an 
annectant form, and therefore must be placed, provisionally at least, 
in a third group by itself. 
The Mystacocetes appear at first sight to be the most specialised 
+and aberrant of the existing Cetacea, as indicated by the absence of 
teeth, the presence of baleen, and the form and size of the mouth ‘ 
but, as we see in other groups, dental characters, and all such as 
