1907.] origin of flight, 231 



Dinosaurs. 

 In consequence of the quite extraordinary tendency of Dinosaurs 

 to specialise every now and then along Avian lines, and in 

 consequence of the fact that the most primitive Dinosaurs are 

 bipedal in their habits, it is not only probable that all Dinosaurs 

 originated from bipedal forms (I onl}' need to quote the numerous 

 bipedal tracks in the Red Triassic Sandstone in Connecticut), but 

 that they also are very nearly related to the primitive Birds. 



Since Dr. Holland thinks that the Dinosaur likeness to Birds 

 is sometimes greatly exaggerated, I would like to mention some of 

 the most characteristic primitive and adaptive Avian features of 

 Dinosaurian reptiles: basis cranii {Hypsilophodon, Compsognathus), 

 development of beak (Orthopoda, beak perhaps developed 

 independently in different suborders, caused by latent homoplasj'), 

 lack of neural spines in cervical vertebrae (Sauropoda), dorsal 

 neural spines bifid [Diplodocus), saddle-shaped articulating surface 

 of sacrals [S'treptospondi/lus), synsacrum (Orthopoda), ^Ep)yornis- 

 like caudals (Diplodocits), Avian scapula (Orthopoda, Theropoda), 

 co-ossification with the coracoid (all Dinosaurs), manus [Orni- 

 tholestes), ilium covering last ribs (Sauropoda), ilium touching- 

 neural spines (Stegosauridte), ilium showing antiti'ochanteric 

 ridge and dorsal plane (Theropoda) ; backward i-otation of pvibis 

 and subsequent development of processus pseudopectinealis 

 (Orthopoda), femur shorter than tibia (many Dinosaurs) ; 

 reduction of fourth trochanter (all Dinosaurs), distal end of 

 femur {Streptospoiidylus, &c.), development of processus ascendens 

 astragali (Theropoda), fusion of calcaneum and astragalus with 

 tibia {Compsognathus) ; position of hallux (Theropoda) ; pneu- 

 maticity or light structure of the whole skeleton (many Dinosaurs). 



In a paper on the evolution of Dinosaurs, I pointed out that 

 the Theropoda specialise by developing an interpubic ossification, 

 by augmenting the number of their sacrals, by changing the 

 character of their vertebrae from biconcave to opisthocoelous, by 

 lengthening their nevu'al spines in the dorsal region, and by 

 dev^eloping a proc. asc. astrag. and reducing the number of their 

 toes. In more specialised Theropoda the metatarsals become 

 always more closely applied, and, lastly, these animals specialise 

 by losing the fourth trochanter. Most of these changes are also 

 notable among the bipedal Orthopoda, and since this develop- 

 ment is independent of that in the Theropoda, we must consider 

 them as homodynamic changes ; besides this, in Orthopoda we 

 can trace a thickening of the bony matter and the development of 

 a processus pseudopectinealis. A functionally analogous osseous 

 process is developed in most running birds after the co- ossification 

 of the pelvic elements. 



Since we can be sure that in Dinosaurs all the changes 

 mentioned are not due to the giving up of volant habits, but are 

 merely signs of cursorial adaptation, we have a clue to under- 

 stand some of the changes that occur among the Palfeognathous 

 Birds, Besides this we can fix the fact that the Dinosaurs, like 



