= 
_ cending one. 
1885, | Progressive and Retrogressive. 345 
which subsequently attained especial development are the wings 
and their appendages; the feet and their envelopes, and the vocal 
organs. Taking all things into consideration the greatest sum of 
progress has been made by the perching birds, whose feet have 
become effective organs for grasping, whose vocal organs are 
most perfect and whose flight is generally good, and often very 
good. In these birds also the circulatory system is most modi- 
fied, in the loss of one of the carotid arteries. 
The power of flight, the especially avian character, has been 
developed most irregularly, as it appears in all the orders in 
especial cases. This is apparent so early as in the Cretaceous 
toothed birds already mentioned. According to Marsh the Hes- 
peornithide have rudimental wings, while these organs are well 
developed in the Ichthyornithide. They are well developed among 
natatorial forms in the albatrosses and frigate pelicans, and in the 
skuas, gulls and terns; among rasorial types the 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 Caprimulgide 
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 Caprimulgide 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 de- 
generation. 
A great many significant points may be observed in the devel- 
opmental 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 insessorial series, 
where those feathers are greatly reduced or absolutely wanting. 
These are cases of degeneracy. 
There is no doubt but that the avian series is in general an as- 
VIII. THE MAMMALIAN LINE. 
Discoveries in palæontology have so far invalidated the ac- 
cepted definitions of the orders of this class that it is difficult to 
