STRUCTURE OF THE VERTEBRATES 169 



mium was placed upon speed. The limbs move in a single antero- 

 posterior plane, and with this digital modifications have ap- 

 peared. An animal on its toes runs more rapidly than one 

 with its feet held flat. All grades are found in the placental 

 mammals. Man, the bears, and many others walk in the planti- 

 grade or flat-footed position, with the heel touching the ground. 

 The carnivores and others have feet adapted for springing, with 

 the weight resting on the ball of the foot, and the heel off the 

 ground. The Artiodactyla and Perissodactyla (page 89) have 

 carried the specialization to its furthest extent. These animals 

 walk on the tips of the digits, only the terminal phalanx touch- 

 ing. This is the ultimate in ease of speedy motion. The evolu- 

 tion of the horse will be found in any biology text. 



4. Arboreal. Tree-living modifications are of many types. 

 The more primitive adaptation has bowed legs for holding the 

 limb or tree trunk, with sharp claws. Among the primates, which 

 have rounded nails, the specializations have been toward swing- 

 ing from limb to limb, rather than a slow climbing. The primate 

 condition has been termed brachiation. Long, well developed 

 arms and fingers are typical of this group. The extreme speciali- 

 zation is found in two genera, one African and one South Amer- 

 ican, where the thumb is vestigial or lacking, and the other four 

 fingers are very long. Mutations of the thumb are not infre- 

 quent, and the selective value of a small thumb is apparent. In 

 jumping from limb to limb the thumb would often interfere with 

 a proper landing; and a sudden fall from a tree-top, or an in- 

 fected thumb from injury, are not conducive to longevity. 



5. Flying. Under flying adaptations may be considered true 

 flight and the volplaning modifications of several groups. Among 

 the latter are the flying squirrels which have a fold of the body 

 skin extending from the front to hind legs. The animal is en- 

 abled to descend at a flat angle and the jumping distance is 

 greatly increased. True flight is found in all the amniote groups. 

 The fossil pterodactyls ("wing fingers") had greatly elongated 

 digits with a leathery flap connecting them. A similar modifica- 

 tion has appeared among the placentals in the bats (page 86). 

 This order has the digits elongated, with a thin membrane con- 

 necting them with the hind legs. The bats have speed, endurance 

 and remarkable coordinations. Flight modifications in the birds 



