262 TOWARDS THE HUMAN FORM 



imply that they had a longevity of five or six centuries. 

 If some retained the squat form, others were slim and 

 elongated. Gradually many of them abandoned the recumbent 

 posture of their ancestors, to which present-day Reptiles are 

 condemned— we shall see why later. 



Using the muscles coupling the limbs to the trunk in a new 

 fashion, the extremities of the humerus and tibia were brought 

 back towards it in such a manner that the arms and thighs, 

 instead of moving in horizontal planes like those of the Lizards, 

 moved in vertical planes like those of the running Mammals. 

 Hence the abdomen no longer trailed on the ground. The same 

 muscular effort exerted on the limbs — now become elongated 

 lever arms — forced the extremity much further forward ; thus 

 the Reptile no longer crawled, but walked, ran, and even jumped. 



Then, assuming an even prouder attitude, the animal raised 

 itself up on its hind legs, which grew very large, whereas the fore 

 limbs, being scarcely used, either shortened or became trans- 

 formed. This was the preparation for the bird form — the form 

 that would very soon venture into and take possession of the air. 

 A certain remarkable anatomical arrangement that was to be 

 retained in the future is to be observed already. Cavities 

 filled with air, no doubt in communication with the lungs, made 

 their appearance either in the vertebrae of the large Dinosaurian 

 Reptiles, 1 or in the long bones, while at the same time the 

 structure and method of using the limbs were already approxi- 

 mating to that of Birds. That it was merely necessary for these 

 Reptiles to make a demand on their muscles for those muscles 

 to bring about such modifications is in no way surprising. 

 A large Australian Lizard of our present day fauna, King's 

 Chlamydosanrus, is still capable of assuming this erect attitude, 

 which is its posture for defence. 



A first attempt at the conquest of the air is thus fore- 

 shadowed, but it was only partially successful. Not until 

 their prey became scarce and the ground a dangerous place 

 for them were Reptiles obliged to hunt in the air, and then 

 only after having been put to tree-climbing in their efforts to 

 find food in security. A further indication of such insecurity 

 on the ground surface lies in the fact that other Reptiles, 

 whose ancestors had taken great pains to leave the water and 

 invade the land, returned to the seas, and, while retaining their 



1 Brontosaiirus, Ccelums, Anchisanrus, Compsognathits. 



