FINS AND LIMBS 321 



have this power in feet as well as hands, while man only 

 preserves the capacity to oppose the first digit in his hands. 



Three separate and independent groups of vertebrates 

 have become adapted to life in the air, by the modification of 

 the fore limbs into wings. These are the extinct Pterosauria 

 (" flying reptiles "), the birds and the bats. In the Pterosauria, 

 the fourth digit of the hand was enormously elongated, and a 

 web of skin was stretched between it and the side of the body, 

 extending back to the hind limbs and tail. The bird's wing is 

 built on an altogether different principle, for the wing-surface 

 is made up of a number of feathers inserted on the hand and 

 forearm. The skeleton of the forelimb of the bird shows a 

 reduction in number of digits to three, and the claws at the 

 end of the digits have disappeared except in the young of some 

 birds, such as the ostrich and of the Hoatzin. The primitive 

 fossil bird Archaeopteryx had well-developed claws. 



The wing of the bat is different, again, for in it the 2nd, 

 3rd, 4th, and 5th digits of the hand are much elongated, and 

 support a web of skin which stretches out from the side of the 

 body. 



The three types of wings just described form another 

 interesting example of convergent evolution on the part of 

 unrelated animals, but the most striking example is that 

 furnished by the limbs of those land-vertebrates which have 

 subsequently returned to an aquatic mode of life and become 

 adapted to it. The adaptation takes the form of a modification 

 of the limbs into flippers or paddles, which superficially may 

 come to resemble the fins of fish, but which betray their 

 descent from the pentadactyl structure of the land-vertebrate's 

 limb in their internal structure. This adaptation has taken 

 place at least nine separate times, in independent groups. 

 Three of these are mammals : the whales, the Sirenia, and the 

 seals. Among the birds, the penguins have modified the 

 wing into a paddle. In the reptiles, the turtles (Chelonia), 

 Ichthyosaurs, Plesiosaurs, Mosasaurs, Thalattosuchia, and 

 Thalattosaurs all show the same modification of the limbs 

 into paddles, and in several fossils it is possible to trace the 

 evolution from normal pentadactyl limbs. 



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