T92 PRIMITIVE FORMS OF LIFE 



•of barbs sustained by a solid axis, which we call the feather. 

 There were feathers on the fore-limbs of those Reptiles which 

 stood erect on their hind legs built for leaping, and these only- 

 needed a certain amount of elongation to support the Reptile 

 in the air and transform it into a Bird. But once the faculty 

 of flight was acquired, the front limbs were modified in their 

 turn, no longer by accident, but by the actual use made of 

 them by the newly achieved bird. To give a greater solidity 

 to the wing during flight the two largest digits were closely 

 pressed against each other. These toes, still quite independent 

 in Archceopteryx, and almost so in the Ostrich, became united 

 and gave the anterior limb the definitive character of a wing. 

 In the same way, in order to assure the free movement of the 

 wing-elevating muscles attached to them, the dorsal vertebrae 

 were united, while the increasing volume of the muscles which 

 lowered the wings brought about the formation between them 

 of a prominent ridge attached to the sternum, called the 

 keel. A kind of epidermal accident has thus affected all 

 the rest of the organism, thanks to the animal's own activity, 

 and determined the direction of its evolution. 



Analogous influences were exerted upon the organization of 

 the limbs by the web developed between the toes of walking 

 Vertebrates which had reverted to an aquatic habitat, but 

 the origin of this web was not accidental, as in the case of the 

 feather. No trace of it exists in any true terrestrial forms. 

 On the other hand, it is observed in all quadrupeds, no matter 

 to what group they belong, which inhabit marshes or water. 

 It is almost universal in the tetrapod Batrachians and occurs 

 again in Crocodiles and marsh or river Tortoises ; it is so 

 characteristic of water Birds that they are called web-footed 

 Birds ; it appears in Mammals, and, in an entirely independent 

 manner, in the Monotreme Ormthorynchus, the Desmans, which 

 -are insectivorous placental Mammals, in Myopotamus,Hydromys 

 and the Beavers, which are Rodents, and in the Martens, Otters, 

 and Seals, which are Carnivores. The fact that a similar con- 

 formation appears in animals so different, whose only common 

 condition of existence is their water habitat, and which is absent 

 in all those which do not share this habitat, clearly indicates 

 that identity in mode of life is the primary cause for the 

 development of the web. Indeed, we can readily understand 

 that contact with the wet ground would soften the skin of the 



