476 BARON FRANCIS NOPCSA ON THE 



and have never consicleied what it explains ; now it is their 

 turn to explain all the points that have been brought forward 

 in the course ©f this paper by means of their hypothesis of an 

 arboreal Proavis. It may be that more difficulties will be 

 encountered than are expected. 



Some critics (12) say that the comparison of a running 

 Proavis with the difierent flying fishes is no comparison at all, 

 because fishes do not move their pectoral fins in flight. In spite 

 of this, I repeat that the comparison is rather a good one 

 because 



(a) both types are vertebrata in which the anterior limbs are 



primarily only used for a similar passive gliding ; 



(b) in both, during gliding the centre of gravity is behind the 



upholding surface, giving it a convenient elevation ; 



(c) in both types the motive power used on land or in water is 



situated at the posterior end of the body and behind the 

 gliding-planes ; 



(d) in both, the locomotive power can immediately be brought 



again into action as soon as gliding through the air ceases, 

 and so gliding can soon be regained. 



ISTone of these four characters can be found in arboreal passive 

 fliers. 



Zscliokke (21) has remarked that some Crustacea develop a 

 sort of flying like that of flying fishes ; so there is no reason why 

 a terrestrial vertebrate animal should not also have been capable 

 of developing it. No mechanical difficulties exist. To those who 

 go on to argue that, with the exception of flying fishes, all other 

 flying vertebrates descended from arboreal animals, and that 

 therefore this was presumably also the case in birds, I retort 

 tha,t, with the exception of flying snakes, all arboreal vertebrata 

 and naturally all their flying offspring are without exception 

 quadrupedal, while this is not the case in birds. 



Literature referred to. 



(1) Abel, 0. — Grundziige der Palseobiologie der Wirbeltiere. 



Stuttgart, 1912. 



(2) Abel, 0. — Lehrbuch der Palseozoologie. Jena, 1922. 



(3) Beebe, 0. W. — Tetrapteryx Stage in the Ancestry of Birds. 



Zoologica, New York, 1915. 



(4) Branca, W. — Fossile Flugtiere und der Erwerb des Flugver- 



mogens. Abhandl. Preuss. Akad. d. Wiss., Berlin, 1908 



(5) DoLLo, L. — Les ancetres des Marsupiaux etaient-ils arbori- 



coles. Miscellanees biologiques dedies au Prof. A. Giard. 

 Paris, 1899. 



(6) Furbringer, M. — Vergleichende Anatoniie des Brustschul- 



tergurtels. Jenaische Zeitschr. f. Naturwiss. 1902. 



(7) Garrod, a. H. — On the Disposition of the deep Plantar 



Tendons in Birds, Proc. Zool, Soc. London, 1875, 



