EVOLUTION OF THE VERTEBRATA. 341 



sand-grouse, and, among the adjacent forms, the pigeons. Then 

 among the lower insessores, the humming-birds exceed all birds 

 in their powers of flight, and the swifts and some of the Capri- 

 mulgidae are highly developed in this respect. Among the higher 

 or true song birds, the swallows form a notable example. With 

 these high specializations occur some remarkable deficiencies. 

 Such are the reduction of the feet in the Caprimulgidae, swifts, 

 and swallows, and the foetal character of the bill in the same 

 families. In the syndactyle families, represented by the king- 

 fishers, the condition of the feet is evidently the result of a pro- 

 cess of degeneration. 



A great many significant points may be observed in the 

 developmental history of the epidermic structures, especially in 

 the feathers. The scale of change in this respect is in general a 

 rising one, though various kinds of exceptions and variations 

 occur. In the development of the rectrices (tail-feathers) there 

 are genera of the wading and rasorial types, and even in the in- 

 sessorial series, where those feathers are greatly reduced or abso- 

 lutely wanting. These are cases of degeneracy. 



There is no doubt that the avian series is in general an as- 

 cending one. 



VIII. THE MAMMALIAN" LIXE. 



Discoveries in paleontology have so far invalidated the ac- 

 cepted definitions of the orders of this class that it is difficult to 

 give a clearly cut analysis, especially from the skeleton alone. 

 The following scheme, therefore, while it expresses the natural 

 groupings and affinities, is defective in that some of the defini- 

 tions are not without exceptions : 



I. A large coracoid bone articulating with the sternum. 

 Marsupial bones ; fibula articulating with proximal end of astragalus 



1. Monotrcmata. 



II, Coracoid a small process co-ossified with the scapula. 



o. Marsupial bones ; palate with perforations (vagina double ; placenta and 

 corpus callosum rudimental or wanting; cerebral hemispheres small 

 and generally smooth). 



But one deciduous molar tooth 2. Marsupialia. 



aa. No marsupial bones; palate entire (one vagina; placenta and corpus 

 callosum well developed). 

 j8. Anterior limb reduced to more or less inflexible paddles, posterior 

 limbs wanting (Mutilata). 



