782 EVOLUTION AND CONSERVATION Part VI 



DEVELOPMENT OF FLIPPER 



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Fig. 38.3. Models of the developing left front flipper of a whale. Note the five 

 digits in the first stage shown, more like a paw or hand than a flipper. In their 

 earlier development the limbs of whales are strikingly like those of their ancestors, 

 the land mammals. Later they become the flippers whose shape is adjusted to swim- 

 ming. Within the flipper, however, the bones are similar in location and relationship 

 to those of the ancestors of whales that lived on land. (Courtesy, British Museum, 

 South Kensington, London, England.) 



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to one another is essentially the same, that is, the parts are homologous in 

 spite of their striking differences in form and function (Fig. 38.3). The basic 

 fore limb of land vertebrates is five-toed and adapted for walking. It has under- 

 gone great changes in different environments and yet has kept a basic plan 

 (Figs. 38.4, 38.5). It may be close to the type, five-toed and soft padded, the 

 silent walking foot of cats; or farther from the type, the single tiptoe running 

 foot of horses (Fig. 38.6); the five-fingered grasping hand of man; the wing 

 of a bird with thumb and first two fingers corresponding to the human hand; 

 the bat's wing supported by four long fingers; and the fleshy flipper of whales 

 and seals (Fig. 38.3). Wings have developed three times during the evolution 

 of vertebrates — in the ancient flying reptiles, in birds, and in bats. They are 

 examples of convergent evolution in the air. The structure arose from the 

 same ancestral stock and retained the same ground plan but differed in ex- 

 pression. In other cases, a water environment offered an opportunity for 

 adaptation in three different classes of animals (Fig. 38.4). 



Racial Long Life. Long ago certain animals reached an almost perfect 

 state of adaptation to environments in which there have been no essential 

 changes. These animals have been unstirred to further evolution. For genera- 

 tion after generation, through millions of years, they have scarcely changed. 

 Among these museum pieces of antiquity are the little reptile Sphenodon (Fig. 

 35.4) which closely resembles the fossils of its ancestors of the Jurassic Period 

 (Table 38.1), the opossum, the "living fossil" Lingula (Fig. 27.15) so like 

 its brachiopod ancestors that are known from their fossil remains of 400 mil- 

 lion years ago, and the common edible oysters very like their ancestors of 



