340 THE STRUCTURAL EVIDENCE OF EVOLUTION. 



true skin. Among the snakes a similar degradation of the organs 

 of sight has taken place in the order of the Scolecophidia, which 

 live underground, and often in ants' nests. The Tortricidse and 

 UropeltidEe are burro wing-snakes which display some of the earlier 

 stages of this process. One genus of the true snakes even (accord- 

 ing to Giinther) has the eyes obscured as completely as those of 

 the inferior types above named (genus Typhlogeophis). 



VII. THE AVIAN LINE. 



The paleontology of the birds not being well known, our con- 

 clusions respecting the character of their evolution must be very 

 incomplete. A few lines of succession are, however, quite ob- 

 vious, and some of them are clearly lines of progress, and others 

 are lines of retrogression. The first bird we know at all com- 

 pletely, is the celebrated Archeopteryx of the Solenhofen slates 

 of the Jurassic period. In its elongate series of caudal vertebrae 

 and the persistent digits of the anterior limbs we have a clear in- 

 dication of the process of change which has produced the true 

 birds, and we can see that it involves a specialization of a very 

 pronounced sort. The later forms described by Seeley and Marsh 

 from the Cretaceous beds of England and North America, some 

 of which have biconcave vertebrae, and all probably, the American 

 forms certainly, possessed teeth. This latter character was evi- 

 dently speedily lost, and others more characteristic of the subclass 

 became the field of developmental change. The parts which sub- 

 sequently 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 modified, 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- 

 peornithidge have rudimental wings, while these organs are well 

 developed in the Ichthyornithidge. They are well developed 

 among natatorial forms in the albatrosses and frigate pelicans, 

 and in the skuas, gulls, and terns ; among rasorial types the 



