GREGORY, THEORIES OF THE ORIGIN OF BIRDS 35 



strongly of the opinion that it was a hopping animal first, and that the 

 metatarsus became strengthened to support the weight of the body en- 

 tirely borne by the hind feet. It is easy to understand a hopping animal 

 taking to an arboreal life and ultimately developing a wing out of a four- 

 toed hand, while it seems unlikely that the hind foot could ever have 

 developed by arboreal habits. It is interesting to note that while the 

 ancestor of the Pterodactyls had four toes in the manus, there is very 

 clear evidence from the skeletogeuesis of the bird that the latter also had 

 a four-toed ancestor. 



'^'^A Pseudosuchian which through a bipedal habit had developed a 

 strengthened ankle-joint and a firm metatarsus, and had lost the 5th 

 digit from the manus would meet all the requirements of the avian an- 

 cestor.'^ ' ' 



The theory that the immediate ancestors of birds were arboreal animals 

 has also been supported by Professor Abel,® of Vienna, who maintained 

 that the birds and carnivorous dinosaurs arose from a common arboreal, 

 ancestral group with climbing feet. The carnivorous dinosaurs soon re- 

 verted to terrestrial habits, while the birds, remaining arboreal, only 

 returned to terrestrial life long after the acquisition of flight. The cleft 

 between birds and carnivorous dinosaurs runs back perhaps to the begin- 

 ning of the Trias. Professor Abel's conclusions rest partly on the facts 

 tliat both the hands and the feet of the smaller dinosaurs show marked 

 resemblances to those of the arboreal bird A rrhrpopferyx. 



Mr. D. M. kS. Watson,' on the other hand, holds that the backwardly 

 directed first toe of some of the earliest dinosaurs was not a perching 

 adaptation, but served as a strut, or prop, for the support of an animal 

 in the early stages of walking upright. 



Quite recently Professor S. W. Williston^ has expressed the belief {in 

 litteris) that the consolidation of the instep bones in the oldest birds was 

 an adaptation to digitigradism, or the habit of raising the body upon the 

 toes, and that fiight originated by leaping from below upward, not by 

 gliding downward. 



Still more recently Mr. C. W. Beebe, of the ISTew York Zoological Park, 

 has made certain discoveries which lend additional evidence for the view 

 that the immediate ancestors of the birds were arboreal animals. In a 

 paper entitled "A Tetrapteryx Stage in the Ancestry of Birds" ^ Mr. 



" Grundziige der Palaobiologie der Wirbeltiere. Stuttgart, 1912. 



■^ '"The Cheirotherium," Geol. Mag., Decade VI, Vol. I, No. 603, September, 1914, pp. 

 395-398. 



* "Trimerorhachis, a Permian Temnospondyl Amphibian," Journ. Geol., Vol. XXIII, 

 No. 3, April-May, 1915. 



« Zoologica, Scientific Contributions of the New York Zoological Society, Vol. II, No. 2, 

 November, 1915. 



