ORIGIN OF FLIGHT IN BIRDS. 



465 



rotation is more or less marked, but tlie hallus is still always well 

 in touch with the ground { Ilerpedaciylus, Xiphopeza, Platy- 

 pterna, Palamopus^ Polemarchus) ; yet in other tracks {Ammoptts, 

 Apatichnus, Anchisauripus, Sauropus) the rotation is still more 

 accentuated, and the articulation is well raised above the ground, 

 so that only the tip of the hallux makes an impression ; and finally 

 in some of the tYSicks.{Gorvipes, Platypterna, Grallator) the hallux 

 can be seen no more. 



Thus in all four types of tracks a simultaneous and indepen- 

 dently acquired rotation and abbreviation of the hallux can be 

 detected. Since it is quite impossible to assume tliat all four 

 types of tracks Avere exckisiveiy made by animals becoming 

 arboreal, it is evident that the curious rotation of the hallux 

 in all these types was not due to arboreal adaptation, but to 

 some other hitherto unknown factor*. 



The hallux that is attained in all these types recalls that 



Text-figure 1. 



Footprints of plantigrade Triassic reptiles. 



(A) Herpedactylus ; (&) Xiplioj>eza\ (G) Fleotopterna ; {D) Plectoptejrna; 

 (E) Corvipes. 



of those birds in which it is a non-functioning digit. If it 

 can be proved that the functioning hallux in birds developed 

 from a uon-functioning hallux, this observation is a strong- 

 argument for a Dinosaur-like terrestrial bipedal running Proavis. 

 If the functioning hallux in birds developed from a non- 

 functioning but rotated and abbreviated hallux, this change 

 must have left some traces. In the functioning hallux of birds 

 two distinct features seem actually to point to such a change. 

 First, one may mention that even in birds with a strong func- 

 tioning hallux the first metatarsal is reduced. This shows that 

 in the history of these birds some stage of evolution was passed 

 in which the hallux was but feebly attached to the leg, and when 

 consequently the hallux was not much used for grasping. 



* Tlie reason why the rotated hallux is move reduced in some Dinosaurs than in 

 others will be discussed on another occasion. 



