BONY SKIN PLATES 34. 



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 \\ay of looking upon 

 the facts is confirmed by the finding, many years ago, l)y the 

 naturalist and physiologist Johannes Muller, 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 Ije 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-limbs do appear in the foetus, a fiict 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-like in form, but the flukes are horizontal 

 instead of vertical as in fishes and Ichthyosaurus. 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-limbs 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 l)eing 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. 



