AND ITS INHABITANTS 129 



tinct adaptation of another sort, that for swift movement, 

 hence the bird-like hind limbs of the dinosaur or perhaps the 

 converse of this statement would be more nearly true. Swift 

 movement, which in this instance implies bipedality, evidently 

 preceded flight, and its impression on the bird was so great 

 that it has never been entirely relinquished despite the attain- 

 ment of the higher faculty. Whether birds passed through an 

 arboreal condition or whether they arose direct from cursorial 

 types is unknown. It hardly seems probable, however, that 

 more than one evolution occurred. We may conceive of a 

 proavian as being at least partially arboreal and launching 

 itself into the air from a convenient tree, sustained for a brief 

 soaring glide on motionless fore limbs and tail, the scales of 

 which had metamorphosed into buoyant feathers (see Fron- 

 tispiece, A). But the first recorded instance, Archaopteryx 

 (Frontispiece, B), of the Upper Jurassic, while still retaining 

 the long lizard-like tail of its reptilian forebears, had already 

 attained the power of sustained flight, though by no means so 

 efficiently as the birds to come. There is again no fossil record 

 of actual transitional forms. 



Origin of dinosaurs. The same primal influence aridity 

 which produced the bird also gave rise to the dinosaur. 

 Whether the latter came within the further influence of glacial 

 cold and evolved a constant temperature of blood is not known; 

 the total absence of any heat-retaining clothing in the dino- 

 saurs, such as hair or feathers, is against such a supposition. 

 That their temperature rose decidedly during their periods of 

 activity is, however, a reasonable assumption. Possibly the 

 early dinosaur-like forms which dwelt within the influence of 

 the Permian cold became the birds, while those beyond its 

 influence remained dinosaurs and as such were destined to 

 dominate the lands as no creatures before them had ever done. 



One is impressed by the influence of aridity in the evolution 

 of bipedal dinosaurs in studying the Upper Triassic footprints 



