I2 4 PRIMARY FACTORS OF ORGANIC EVOLUTION. 



trogression. The first bird we know at all completely, 

 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 indication 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 Amer- 

 ica, some of which have biconcave vertebras, and all 

 probably, the American forms certainly, possessed 

 teeth. This latter character was evidently speedily 

 lost, and others more characteristic of the subclass be- 

 came the field of developmental change. The parts 

 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 be- 

 come 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 circula- 

 tory system is most modified, in the loss of one of the 

 carotid arteries. 



The power of flight, the especially avian charac- 

 ter, 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 Hesperornithidae 

 have rudimental wings, while these organs are well 

 developed in the Ichthyornithidae. They are well de- 

 veloped among natatorial forms in the albatrosses and 

 frigate pelicans, and in the skuas, gulls, and terns, 

 and are rudimental in their allies, the auks. They are 



