126 PRIMARY FACTORS OF ORGANIC EVOLUTION. 



some of the Caprimulgidae 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 CaprimulgidaB, 

 swifts, and swallows, and the foetal character of the 

 bill in the same families. In the syndactyle families, 

 represented by the kingfishers, the condition of the 

 feet is evidently the result of a process of degenera- 

 tion. 



A great many significant points may be observed 

 in the developmental history of the epidermic struc- 

 tures, especially in the feathers. The scale of change 

 in this respect is in general a rising one, though vari- 

 ous 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 insessorial series, where those feathers are of prim- 

 itive structure (Menuridae), are greatly reduced, or 

 absolutely wanting. These are cases of degeneracy. 



There is no doubt that the avian series is in gen- 

 eral an ascending one. 



f. The Mammalian Line. 



Discoveries in paleontology have so far invalidated 

 the accepted 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, there- 

 fore, while it expresses the natural groupings and affin- 

 ities, is defective, in that some of the definitions are 

 not without exceptions : J 



IThis classification of the Mammalia was first published by the writer 

 in the American Naturalist for 1885 ; was improved in the same, 1889 (October); 

 and appeared in a Syllabus of Lectures of the University of Pennsylvania, 

 July, 1891. 



