56 A HISTORY OF BIRDS 



The characters which the Pigeons and Sand-grouse share in 

 common with the Plover tribe are indicative of a common 

 descent. The several stems, in short, represent the splitting up 

 of a common ancestral stock, indicated in the diagram by B 

 II., I, and this sufficiently explains the fact that the Pigeons 

 would seem to incline more towards the Galli than towards 

 the Plovers in the matter of intestinal convolutions. 



We have arrived now at a very critical point in the problem 

 of avian descent — the question of the relationship of the " Cora- 

 ciiform " birds to the " Passeres," and the position of both 

 with regard to the rest of the Neognathine birds. 



As to the wider question : the weight of evidence seems to 

 show that the members of this great group, collectively known 

 as the Coraciomorphse, must be regarded not so much as an 

 offshoot of any other of the groups so far considered, as an 

 independent stem of great antiquity : a derivative of that 

 generalised stock whose descendants represent the Neognathine 

 types so far reviewed. Yet there seems to be some sort of 

 affinity, though remote, between the Coraciomorphae and the 

 Gallinaceous birds, as if the two were to be regarded as having 

 had a common origin, such as is indicated in the diagram B II., 

 2. If it be admitted that this interpretation is correct, there 

 still remains for settlement the position of the Passeres. So 

 far the evidence seems to show that they are to be regarded 

 as derivatives of a Caprimulgine stock. But to this point we 

 must refer again after passing in review the principal groups 

 of the Coraciomorphae. 



Main Lines of Evolution 



Before proceeding to the further sections of this work 

 it would be well to briefly review the main lines of evolu- 

 tion which the avian stock has undergone. The earliest bird 

 known, it will be remembered, was of a strictly arboreal type, 

 and we have shown that it is almost certain that this had been 

 evolved from an arboreal reptile stock. In other words, the 

 Class Aves began its development as a forest haunting tribe. 

 From this environment it appears to have spread to more open 

 country. This migration must have been followed by material 

 changes of form in adaptation to the new environment, the 

 avoidance of new enemies, the capture of new food, the forma- 



