ORIGIN OF FLIGHT IN BIRDS. 475 



and extinct birds not a single chai'acter can be detected which 

 points conclusively towards a primitive arboreal adaptation, but 

 that several skeletal characters can only be explained by the 

 hypothesis of a cursorial running ancestor of bii'ds. The other- 

 wise inexplicable features are : the fused median metatarsals, 

 the reduction of the first metatarsus, the small tarsal bones, 

 the position of the knee, the long symphysis of the pubes in 

 ArckcBopter.yx, the lack of clavicles and interclavicle in all birds, 

 and the position of the occipital condyle. The latter is vertical 

 in birds and primitive Dinosaui's, a.nd proves that, unlike 

 arboreal forms, in both these long-necked groups the head was 

 primarily carried upright on a sigmoida,l neck. 



Apart from all these important points, it is only by means of 

 the YwwMva^-Proavis hypothesis that the early occurrence of 

 numerous flightless birds can be explained. It is evident that 

 badly flying ground birds will much more i-eadily return to 

 exclusively cursorial habits than arboreal birds, for these, before 

 becoming again flightless, must first descend from their trees and 

 become thoroughly adapted to ground life. There exists a 

 great difierenee between the pelvis of formerly arboreal flightless 

 birds (for example. Dodo) and that of the Palajognathse. 



The hypothesis of a running Proavis is also the single one that 

 accounts for the primitive structure of the palate, the free dorsal 

 vertebrse, the low body-temperature, and the strong penis of the 

 greater part of the Pal8eogna,tha3, for according to this hypo- 

 thesis these birds are, of course, much nearer I'elated to Proavis 

 than the later and evidently more specialised aboreal birds. If 

 the birds were of arboreal origin, one would expect the most 

 primitive birds among the arboreal and not the terrestrial birds. 

 In compliance with a wish of Steiner(17), I desist from using in 

 the course of these comparisons the term Ratitfe. 



That in one primitive arboreal bird more specialised, however, 

 than an ostrich (i. e. Opisthocomus) the claws of the wing are 

 more used for climbing than in all the other recent birds, is not 

 of very great weight, for other nestlings use even their beak 

 when climbing, and yet nobody will consider the PaiTots to be 

 ancestral to all the other birds. The climbing of Opisthocomus 

 may quite well be regarded as a secondary adaptation, for the 

 development of the pinions of the wings in Opisthocomus suff"ers 

 a cvirious retardation that points in this direction (14). The 

 sternum of Opisthocomics shows likewise a curious modification. 



The primitive structure of the nest of ground birds when 

 compared with the nest of the arboreal birds, and the more 

 brilliant coloration of the latter, also agree very well with the 

 hypothesis of the terrestrial origin of all birds. Even the 

 curious fact that the nestlings of ground biixls as a rule leave 

 their nest comparatively soon after birth seems to some extent 

 to favour this' hypothesis (14). 



Up to the present all critics of the "running Proavis" hypothesis 

 have only tried to find apparent difficulties in that hypothesis, 



