ORIGIN OP FLIGHT IN BIRDS. 



469 



mammals and lizar;ds, not only do not form an argument for the 

 arboreal origin of birds and Dinosaurs but on the contrary plead 

 for their origin from running ancestors, and after having, as I 

 believe, refuted the arguments brought forward by Abel, Hay, and 

 Sfceiner, I think the next step should be to mention all those 

 points in the anatomy of birds which exclude an arboreal Proavis 

 frqna the history of this group. 



All birds have a fused metatarsal. Where it is but feebly 

 fused, as in the Penguins, this is a return to a, primitive stage, for 

 in the fossil Penguins of the Seymour Islands the metatarsals are 

 more strongly fused than in any recent species (20). 



With exception of arboreal birds, not a single arboreal verte- 

 brate animal is known in which a fusion of the metatarsals is 

 even indicated. This fusion shows that Proavis must have 

 moved differently from all recent and fossil arboreal forms. 



Text-figure 5. 



A B ]v<ex C 



Bones of manus in Dinosaurs and birds. 

 {A) Massospondylas; (B) Antrodesmus ; (C) Struthiomimus ; (D) Ornitholestes ; 

 (E) Archceopteryx. 



Proavis cannot have leaped in a more or less frog-like manner like 

 Tarsitos, Galago, or frogs, for in such arboreal animals the 

 flexible tarsus is modified and its proximal part elongated ; it 

 cannot have climbed, for the manus is specialised on other lines ; 

 it cannot have ju.mped like a monkey, for birds show no tuber on 

 the heel. Even Pecletes, though terrestrial and provided with com- 

 pact tarsal bones, shows how even the firmly united tarsal bones 

 are finally alfected by jumping, for in the recent Pecletes these 

 bones are somewhat elongated, while they are shoi't in a more 

 primitive fossil type that Prof. Stromer intends to describe. 

 This elongation of the tarsal bones is necessary in jumping 

 animals, for in these it is essential that that part of the pes 

 should be rigid and long which immediately touches the tibia, 

 and not the part beyond the flexible tarsal bones. In bipedal 

 stalking and walking animals the metatarsals can moi*e easily 

 elongate so as to ensure a greater stride. 



Proc. Zool. See— 1923, No. XXXI. 31 



