CHIEF CHARACTERISTICS 3 



sional skimming leaps, helped by strokes of the somewhat 

 webbed fore-limbs and balanced by the outstretched tail. 

 It is more than a guess that birds had to walk as bipeds 

 before they could run, and that they had to leap before 

 they could fly. For the palaeontological facts, to be dis- 

 cussed afterwards, point to the extinct Dinosaurs as the 

 stock from which birds evolved, and point particularly to 

 a sub-order known as the Ornithischia, some of which 

 were bipeds. Thus there is a basis of fact for the view 

 that the primitive birds were bipeds before they were fliers. 

 It is interesting to find in the Australian Collared Lizard 

 {Chlamydosaurus) an animal which at the present day is 

 making essays in bipedal progression. It runs a short 

 distance almost erect, and then topples down on all fours 

 again. After a period of terrestrial life, birds became no 

 doubt arboreal, and after a period of arboreal apprentice- 

 ship in which the fore-limbs were mainly used as parachutes 

 they acquired true wings — striking the air. The point to 

 be considered is that the assumption of a bipedal mode of 

 progression, indicated by the extinct Ornithischia, implied 

 an emancipation of the fore-limb from the task of being a 

 supporting organ, and opened up the possibility of a new 

 function, namely, flight. 



§ 3. Supply of Body with Arterial Blood 

 Exclusively 



It is not a wild fancy to suppose that before birds were 

 fliers they had acquired a four-chambered heart and other 

 arrangements that secured a supply of nothing but arterial 

 blood to the body. For that would mean a raising of the 

 pitch of the whole life. Here it must be remembered that 

 in most reptiles there is an incomplete septum up the 

 middle of the driving chamber or ventricle of the heart, so 

 that the impure blood which comes in from the body via the 

 right auricle is not completely shut off from the pure blood 

 which comes in from the lungs via the left auricle. It is 

 not a very big anatomical advance to have this ventricular 



