50 THE MAMMALIA. 



whale ; and that it once possessed the ordinary Mammalian 

 dentition is shown by the foetal teeth of the unborn whale, which 

 are absorbed before birth. Yet, this most highly specialised 

 animal, which, upon any theory of evolution, must have taken 

 thousands upon thousands of years for its profound modifications 

 of form, is yet found in the oldest Tertiary beds ; possibly, in 

 rocks of the upper Jurassic age {FalcBocefiis Sedgwickii). The 

 oldest Cetaceans, the Zeuglodons, have a much less highly 

 specialised dentition than the modern whales. They have strong 

 molar teeth, with serrated crowns, sometimes ten in number, and 

 implanted in the jaw by two roots, incisors, and a deciduous den- 

 tition. But the profound modifications of the hinder portions of 

 the skeleton remain the same. The whales are also peculiarly 

 refractory to the special-creation hypothesis, for it is impossible to 

 imagine an animal created with teeth in the foetal state only and a 

 rudimentary pelvis and femur. 



The Sirenians, which have many points of affinity with the 

 Cetaceans, are rapidly dying out. The highly-modified form, 

 Rhytina, possessing no true teeth, was exterminated about the 

 middle of the eighteenth century. All the Sirenia possess a rudi- 

 mentary pelvis, but it has become detached from the vertebral 

 column, and the rudimentary thigh-bone has wholly disappeared. 

 Halitherium, a Sirenian of the Miocene, still possessed a 

 rudimentary femur. The Sirenians appear, like the whales, in the 

 Eocene period. Unlike the Ungulata and the Carnivora, which 

 can be traced back step by step to more generalised forms, the 

 Sirenia and Cetacea appear suddenly before us at the earliest 

 Tertiary period, wuth their profound modifications of form. The 

 comparative anatomist is almost forced to the conclusion that the 

 rocks of some long geological age are somewhere concealed 

 beneath the sea — perhaps lost to our eyes for ever, only to be 

 descried by the scientific imagination. 



Not only the Cetacea and Sirenia, but other highly specialised 

 forms appear suddenly before us in the Eocene. The bats, the 

 most deeply modified of all the Insectivora (and not only the 

 sub-order bats, but a form closely similar to t/ic exisfi?ig Eu?vpean 

 bat), appear in the Eocene of Europe. The Edentata (sloths, 

 armadillos, etc.) have been found no further back than the 



