40 LIFE ON THE EARTH. 



doubling and cross-folding of the membranous wings 

 under their waterproof horny elytra, as a singular 

 and expressive example of a purely mechanical ope- 

 ration, performed on an organ for its conservation, 

 more ingenious than the putting-up of a fan in its 

 varnished case, or a parapluie into its oiled sheath. 



The same mechanical principles may be traced in 

 the wings of the Bat, carried out in practice in such 

 a manner as to suit the Mammalian type of bones 

 and dermal covering. The fluttering collapse of the 

 soft skin of the Vespertilionidse is however a much 

 lower order of movement in air than the rapid sweep 

 of the elastic feather-wing of the bird ; nor need we 

 suppose for the flying Lizards of the Mesozoic ages a 

 very rapid or very sustained flight, notwithstanding 

 the hollowness of their elongated digital bones, the 

 carinated sternum, and the devotion of a large part 

 of the muscular energy to the anterior limbs. But 

 there is no reason whatever for comparing the Pte- 

 rodactyle with such parachute-bearing Mammals as 

 Petaurus, and Pteromys, and still less with the Lizard 

 called Draco volans. The membranes extended be- 

 tween the limbs of the former creatures, and beyond 

 the ribs of the latter, serve only to retard their fall, 

 and thus to allow of their making longer leaps, espe- 

 cially among trees, just as the flying-fish makes long 

 leaps by the sustaining power of its expanded pec- 



