170 Dynamic Theory. 



with hair and appears to be simply a fold of their ordinary skin project- 

 ing from each side. This arrangement makes the little animal nearly 

 three times as wide, and enables him to make long flying leaps. 



On their feet they have long bony appendages which assist in support- 

 ing these expansions of skin. 



There can be no doubt that this flying apparatus, including the bony 

 supports, have been developed by the leaping habits of the animal. In 

 much the same way were the side and median folds of the skin in fishes 

 developed, which subsequently became specialized into dorsal, ventral, 

 lateral and tail fins. 



The development of limbs was therefore independent of the backbone, 

 and their attachment to it was a later arrangement. In fact, the scap- 

 ula and pectoral arch with the fore limbs do not directly articulate with, 

 the spine, although the pelvic bones usually do. 



In fishes, however, the pelvis bones with hind limb are seldom 

 attached to the spine, but are often placed in advance of the belly and 

 attached to the bones of the shoulders. Sometimes the pectoral fins are 

 absent and then the bones of the pectoral arch are rudimentary and of 

 little or no use. 



In tortoises the development of the carapace or shell made an excep- 

 tional arrangement of the bones possible. So the coracoid is wanting 

 and the shoulder blade reaches from the carapace to the sternum. That 

 is, there was sufficient rigidity without the coracoid, and so it became 

 obliterated. The scapula and clavicle appear to be fused together. In 

 the true Serpent, on the other hand, there is no shoulder blade, clavicle, 

 coracoid nor sternum. This animal moves without limbs by means of 

 the alternate expansion and contraction of its belly muscles. Such 

 motion as that would be prohibited by such rigid pieces as the sternum, 

 the marsupial bones and ventral plates. 



This animal, as well as the Mammal, is a descendant of reptiles with 

 limbs. But while one branch of such reptile stock advanced by using 

 its limbs to lift it from the earth, the other took the retrograde track 

 and suppressed its limbs in its endeavors to hug the earth still closer. 

 The advance of the one would compel the retrogression of the other, 

 whose only safety remained in Its becoming inconspicuous, unaggres- 

 sive, cautious and sly, in the same ratio in which the advancing branch 

 became active, enterprising and aggressive. It is so in human society, 

 too. One wing cannot advance greatly beyond the average without 

 entailing a retrogressive movement on another one. And so the snakes., 

 by the compulsory disuse of their limbs, have lost them. 



