6o A HISTORY OF BIRDS 



From the Caprimulgine stem there arose a branch which 

 gave rise to the Striges (Owls), Cypseli (Swifts and Humming- 

 birds), CoHi (Mouse-birds) and the " Passeres ". 



The Coraciomorphine stem should probably be regarded 

 as extremely ancient. It is highly probable indeed that we 

 should be near the truth, as we have already hinted, in consider- 

 ing it as arising from the same generalised stock as that which 

 gave rise to the Pelargocolymbomorphine, and Alectoromor- 

 phine trunks. 



Finally, the gradual splitting up and differentiation of this 

 great Coraciomorphine stem has been largely due to adaptation 

 and to the pursuit of food. And these adaptations we shall 

 discuss in due coarse, at least in their more salient features. 



Numerical Strength 



In the matter of numbers the Class Aves far outnumbers all 

 other vertebrates, since about 19,000 species are now known to 

 science. It is significant to note that the Fishes approach 

 nearest to the Birds in this matter, since between 8,000 and 9,000 

 living species are known. Of the Amphibia there are probably 

 not more than 1,000 species. Of Reptiles some 3,500 have 

 been described, and of Mammals between 2,000 and 3,000. 



This numerical superiority of the Birds and Fishes must be 

 ascribed to the comparative absence of barriers to their dis- 

 tribution. 



As to their past history there is little to be said, for the 

 records of the rocks have revealed less on this head than on 

 any other group of vertebrates. Such fossils as are of import- 

 ance, however, are described later in this work. 



