DIVISIONS OF BIRDS. 499 



Before passing on to the consideration of the divisions of 

 Birds, a few words may be said as to the migration of birds. 

 In temperate and cold climates comparatively few birds remain 

 constantly in the same region in which they were hatched. 

 Those which do so remain, are called " permanent birds " (aves 

 manentes). Other birds, such as the Woodpeckers, wander 

 about from place to place, without having any fixed direction. 

 These are called " wandering birds " (aves erratic^}, and their 

 irregular movements are chiefly conditioned by the scarcity or 

 abundance of food in any particular locality. Other birds, 

 however, at certain seasons of the year undertake long jour- 

 neys, usually uniting for this purpose into large flocks. These 

 birds such as the swallows, for instance are properly called 

 " migratory birds " (aves migrator itz). The movements of 

 these birds are conditioned by the necessity of having a cer- 

 tain mean temperature, and consequently they leave the cold 

 regions at the approach of winter, and return again for the 

 warmer season. 



CHAPTER LXVII. 

 DIVISIONS OF BIRDS. 



i. GENERAL DIVISIONS OF AVES. 2. NATATORES. 

 3. GRALLATORES. 



OWING to the extreme compactness and homogeneity of the 

 entire class Aves, conditioned mainly by their adaptation to an 

 aerial mode of life, the subject of their classification has been 

 one of the greatest difficulties of the systematic Zoologist. 



By Professor Huxley the Birds are divided into the following 

 three orders : 



1. SAURUR^. In this order the caudal vertebrae are nume- 

 rous, and there is no ploughshare-bone. The tail is longer 

 than the body, and the metacarpal bones are not anchylosed 

 together. This order includes only the single extinct bird the 

 ArchtKOpteryx macrura, in which the long lizard-like tail is only 

 the most striking of several abnormalities. 



2. RATIIVE. This order comprises the Running birds, 



should consult the masterly article by Owen on " Aves " in the " Cyclo- 

 paedia of Anatomy and Physiology," or the second volume of the " Verte- 

 brata " of the same author, from which the preceding summary has been 

 chiefly derived. 



