614 PROCEEDINGS OF THE NATIONAL MUSEUM vol. io4 



the result that the predecessors for all major avian and mammalian 

 groups might well have been developed. Subsequent to their origin 

 and early radiation, further evolution of the major groups could have 

 taken place in other and more widely separated areas with the re- 

 sultant complex distribution of forms, both fossil and living. 



On the assumption that evolution is greatly accelerated in a tropical 

 climate and that the arboreal habit in a tropical climate tends to be 

 predominant, it may be concluded that ancestral primates and 

 ancestral marsupials were more or less contemporaries (Simpson, 

 1945), and that the eutherians were not evolved from the metather- 

 ians but that both groups have evolved independently and simul- 

 taneously along somewhat different and more specialized lines (Romer, 

 1945). The same view then may be taken with respect to the evolu- 

 tion of the major orders of birds. Thus, once the earliest avian 

 ancestors had become established, the pro-order and perhaps even the 

 profamily types probably developed quite rapidly and these may weU 

 have been contemporaries. Since the earliest beginnings of proavian 

 forms there has been a notable degree of specialization of structures, 

 feeding habits, and environmental adjustments resulting in the com- 

 plex avian fauna as we know it today. 



References 



Amadon, Dean 



1942. Birds collected during the Whitney South Sea Expedition. L. Amer. 

 Mus. Novit. (1942), No. 1176, pp. 1-21. 

 Bakst, Henkt J., and Chafee, F. H. 



1928. The origin of the definitive subclavian artery in the chick embryo. 

 Anat. Rec, vol. 38, pp. 129-140. 

 Balfour, Francis M. 



1873. The develoioment of the blood-vessels of the chick. Quart. Journ. 

 Micr. Sci., new ser., vol. 13, pp. 280-290. 

 Barkow, Hans C. L. 



1843. Disquisitiones recentiores de arteriis niammalium et avium. Nova 

 Acta Leop.-Carol. Akad. Naturf., Halle, vol. 20, pt. 2, pp. 607-720. 

 Bauer, Friedrich 



1825. Disquisitiones circa nonnullarum avium systema arteriosum. Inaug. 

 Diss. Berolini, 24 pp. 

 Beddard, Frank E. 



1898. The structure and classification of birds. 



1905. A contribution to the knowledge of the arteries of the brain in the 

 class Aves. Proc. Zool. Soc. London (1905), pt. 1, pp. 102-117, 

 Bhaduri, Jnanendra Lal 



1930. Notes on the arterial system of the common Indian toad Bufo mel- 

 anostidus Schneid. Journ. Proc. Asiatic Soc. Bengal, new ser., 

 vol. 24, pt. 1, pp. 301-315, 

 1939. A case of persistence of ductus caroticus in the Indian blue rock- 

 pigeon Columba livia intermedia (Strick.). Anat. Anz., vol. 88, 

 pp. 178-182. 



