ORIGIN OP FLIGHT IN BIRDS. 



473 



taken up an arboreal life, tliey must have developed feathers 

 on arms and legs, for also in these long -necked and long-tailed 

 forms the centre of the supporting gliding surface had to coincide 

 with the centre of gravity of the body. Hence, evidently out of 

 these reptiles a Tetrapteryx had to develop. 



Of course an exclusively bipedal Tetrapteryx vi^ould encounter 

 great difficulties when climbing. During gliding, furthermore, 

 in spite of new difficulties presenting themselves, the knees of such 

 a creature had evidently to be turned outward ; the feet had to 

 be stretched away from the body and had to be brought into the 

 same plane as the anterior members, else the feathers of the legs 

 would have been quite ineffective. All this and the difficulties 

 encountered when climbing without using the anterior limbs 

 would have been very detrimental to bipedal locomotion. Even 

 a mere glance at Beebe's hypothetical Tetrapteryx shows that such 

 a creature could not have been bipedal. 



Since Beebe's Tetrapteryx is a i-ather short-necked animal, this 

 point of his drawing is likewise open to criticism. All birds 

 originated evidently from a bipedal and long-necked ancestor 

 that carried the head upright. It will, however, always remain 

 an unsolved problem how Tetrapteryx could have managed to 

 carry such a long neck and an upright head when gliding : 

 arboreal animals are rather short-necked. 



In consequence of all this, the Tetrapteryx hypothesis, which 

 is but a revival of Py craft's arboreal Proavis, must evidently be 

 dismissed, for it can in no way account for the persistence of 

 bipedal locomotion in birds. 



The whole Tetrapteryx hypothesis is based exclusively on 

 tlie one interesting observation, that in some birds a row of 

 quills is present on the upper part of the legs. 



Without diminishing the interest of this observation, it has to 

 be remarked that in a long-tailed animal this is the very place 

 where quills would be ineffective, and that the phylogenetical 

 value of this observation varies according to the manner in which 

 one estimates the relation between quills and down. 



If the down is considered to be the primitive type of feather 

 which originated as a body cover to prevent the loss of heat, and 

 the quills are considered as its modification, of course in such a 

 case the occurrence of a row of quills on the leg can be taken as 

 a proof that the legs were once used for flying ; but if, on the 

 other hand, the quills are considered as primitive, then such a 

 reasoning is fallacious. In this latter case the occurrence of 

 quills in places that are later covered with down possibly only 

 shows that formerly a part of the side of the body of the primitive 

 birds was covered with quills arranged in rows. Such quills can 

 then gradually have been replaced by down, except in places 

 where they served for flight or steering. 



In such a case it may be assumed that the quills which 

 evidently first originated on the arms and tail gradually spread 

 to adjacent places where they were not really wanted. 



