XII BONY SKIN PLATES 343 
it has been discovered, are larger in the foetus, a fact which 
naturally points to their being an inheritance from the past, now 
undergoing retrogressive changes. Such a way of looking upon 
the facts 1s confirmed by the finding, many years ago, by the 
naturalist and physiologist Johannes Miiller, of bony plates in 
connexion with the remains of a Zeuglodont Cetacean. It looks, 
therefore, very much as if the Eocene ancestors of the modern 
Cetacea had a skin studded with bony plates, as have the arma- 
dillos. This being the case, the disappearance of hair is not 
surprising. The room would be taken up by the calcified plates, 
and when the latter disappeared, as they have in the vast majority 
of existing Whales, the naked skin alone would be left. 
Whales possess no externally-visible hind-limbs ; rudiments 
of these appendages are present, which will be dealt with under 
the description of the principal features of the skeleton. But 
it has been discovered that in the Porpoise, external vestiges 
of hind-lmbs do appear in the foetus, a fact which, be it ob- 
served, does away with the old view that the flukes of the Whale 
are the last term in the series of vanishing hind-limbs, of which 
the Seals, with their hind-limbs and tail bound up together, offer 
an intermediate step. 
The tail is fish-lke in form, but the flukes are horizontal 
instead of vertical as in fishes and Jchthyosaurus. This arrange- 
ment is no doubt associated with the need for rapid return to 
the surface waters after a prolonged immersion in search of food. 
A downward stroke, such as is given by the powerful and large 
tail flukes, would naturally bring about this result rapidly. 
The tail, moreover, is under all circumstances the swimming 
organ. Its motion has been stated to be slightly rotatory, like 
that of a screw, and it is the case that the two flukes are often 
alternate in shape like the flanges of a screw; one being convex 
upwards, the other convex downwards. 
The fore-lmbs are in the form of paddles, but they do not 
apparently serve as organs of locomotion so much as_balancers. 
When a Whale is killed, it falls over on to one side, the office of 
the flippers being to maintain the proper position. It is be- 
lieved, however, from the fact that the embryo often shows a 
relatively larger pectoral fin than that of the adult—the differ- 
ence being due to a reduction in the adult of the number of 
phalanges—that the fin was once an organ of progression. 
