160 STRUCTURE AND CLASSIFICATION OF BIRDS 



Now this at once lands us in a difficulty, which has been 

 too lightly regarded by many systematists. Phylogenetic 

 schemes used to be boldly linear, and even so recently as 

 the attempt of FUEBBINGER the family tree savours a little 

 too much of the linear arrangement. Now the imperfect 

 remains of birds that have come down to us from tertiary 

 times show that the modern types of birds were fully 

 differentiated even then in addition to a few extinct forms, 

 such as Odontopteryx toliapicus (if this benot asteganopod). 

 But beyond that point there is the most scanty record of 

 bird life, limited to the Cretaceous Ichthyornithidae and 

 Hesperornithida?,, with a few obscurer forms, and to the 

 Jurassic Archceopteryx. So emphatically were all these 

 creatures birds that the actual origin of Aves is barely hinted 

 at in the structure of these remarkable remains. Moreover, 

 at least in the case of Ichthyoniis, they depart fully as 

 widely from any bird with the required ' mixed ' characters 

 as any living group, while Hesperornis can with safety be 

 relegated to the neighbourhood of the existing divers. We 

 get, therefore, no help whatever from the Cretaceous birds, 

 and, if any, only the scantiest assistance from Archteopteryx, 

 in determining what are archaic characters in birds. There 

 are no criteria by which we can assert with any degree of 

 safety the relative positions of this and that existing group ; 

 nor has the study of the comparative embryology of birds 

 as yet advanced sufficiently far to give any results, except 

 in isolated characters ; such indications are the relatively 

 primitive character of the basipterygoid processes, at least 

 in certain groups ; for the gulls which are without them 

 when adult, have them as young chicks, as have in the 

 adult condition most of their near allies, the Limicolse. It 

 may, therefore, so far be inferred that the gulls are a 

 modification of the limicoline type and not vice versa. 



It would be perhaps held that any type in which a 

 number of undoubtedly reptilian characters had survived 

 would be on a lower level of organisation than other types 

 in which fewer or no such characters could be discovered. 

 But the few specially reptilian features in the organisation 



