INTRODUCTION. 



35 



expansion occurring between the toes, are supplied with a 

 powerful swimming-tail, which adds very materially to the 

 surface engaged in natation. Those animals, one and all, 

 walk awkwardly, it always happening that when the ex- 

 tremities are modified to operate upon two essentially 

 different media (as, for instance, the land and water), the 

 maximum of speed is attained in neither. For this reason 

 those animals which swim the best, walk, as a rule, with the 

 greatest difficulty, and vice versd, as the movements of the 

 auk and seal in and out of the water amply testify. 

 , In addition to those land animals which run and swim, 

 there are some which precipitate themselves, parachute- 

 fashion, from immense heights, and others which even fly. 

 In these the membranous expansions are greatly increased, 

 the ribs aflbrding the necessary support in the Dragon or- 

 Flying Lizard (fig. 15), the anterior and posterior extremities 

 and tail, in the Flying Lemur (fig. 16) and Bat (fig. 17, p. 36). 



Fig. 15. Fig. 16. 



Fig. 15. — The Ked-throated Dragon (Draco TifEmatopogon, Gray) shows a large 

 membranous expansion (b h) situated between the anterior (d d) and pos- 

 terior extremities, and supported by the ribs. The dragon by this arrange- 

 ment can take extensive leaps with "perfect safety.— Ongfma?. 



Fig. 16. — The Flying Lemur [Gahoplthecus volans, Shaw). In the flying 

 lemur the membranous expansion (a h) is more extensiA'e than in the 

 Flying Dragon (fig. 15). It is supported by tlie neck, back, and tail, and 

 by the anterior and posterior extremities. The flying lemur takes enor- 

 mous leaps ; its membranous tunic all but enabling it to fly. The Bat, 

 Phyllorhina gracilis (fig. 17), flies with a very slight increase of surface. 

 The surface exposed by the bat exceeds that displayed by many insects 



