EVOLUTION OF THE YERTEBRATA. 349 



seen in the Hyaenidge and the Felidae, are the latest of their line 

 also. 



Some cases of degeneracy are, however, apparent in the mono- 

 delphous Mammalia. The loss of pelvis and posterior limbs in 

 the two mutilate orders is clearly a degenerate character, since 

 there can be no doubt that they have descended from forms 

 with those parts of the skeleton present. The reduction of flexi- 

 bility seen in the limbs of the Sirenia and the loss of this char- 

 acter in the fore limbs of the Cetacea are features of degeneracy 

 for the same reason. The teeth in both orders have undergone 

 degenerate evolution ; in the later and existing forms of the Ceta- 

 cea even to extinction. The Edentata appear to have undergone 

 degeneration. This is chiefly ajoparent in the teeth, which are 

 deprived of enamel, and which are wanting from the premaxillary 

 bone. A suborder of the Bunotheria, the Tgeniodonta of the 

 Lower Eocene period, display a great reduction of enamel on the 

 molar teeth, so that in much-worn examjoles it appears to be 

 wanting. Its place is taken by an extensive coat of cementum, 

 as is seen in Edentata, and the roots of the teeth are often un- 

 divided as in that order. It is probable that the Edentata are the 

 descendants of the Tseniodonta by a process of degeneracy. 



Local or sporadic cases of degenerate loss of parts are seen in 

 various parts of the mammalian series, such as toothless jMamma- 

 lia wherever they occur. Such are cases where the teeth become 

 extremely simple, as in the honey-eating marsupial Tarsipes, the 

 carnivore Proteles, the Pteropod bats, and the aye-aye. Also where 

 teeth are lost from the series, as in the canine genus Dysodus, and 

 in man. The loss of the hallux and pollex without corresponding 

 gain in various genera, may be regarded in the same light. 



In conclusion, the progressive may be compared with the re- 

 trogressive evolution of the Vertebrata, as follows : In the earlier 

 periods and with the lower forms, retrogressive evolution pre- 

 dominated. In the higher classes progressive evolution has pre- 

 dominated. When we consider the history of the first class of 

 vertebrates, the Tunicata, in this respect, and compare it with 

 that of the last class, the Mammalia, the contrast is very great. 



