The Structure of Wings 259 



sions of the sides of the body, as in insects, although similar expansions may 

 be developed in connection with the limbs in the animals we are now to 

 consider. 



In the flying fish, which inhabit the warmer seas of the globe, the pec- 

 toral fins are greatly enlarged, and enable the animals to leave the water and 

 to fly through the air a short distance when hotly pursued by their enemies. 

 The flying fish is not a great master of the art of flight, and can hardly be 

 compared in this respect with birds, bats and pterodactyls. 



The pterodactyls were reptiles which, fortunately, perhaps, for us, be- 

 came extinct many ages ago — in what is known by geologists as the Mesozoic 

 period. Had we been living at that time we should not have feared the 

 smaller species, which were only about as large as sparrows, but the large 

 pterodactyls, with their long, toothed jaws and wings twenty -five feet across 

 when expanded, would have been far more formidable. In the pterodactyl, 

 the wing consisted of a large membrane extending from the posterior sur- 

 face of the arm and enormously lengthened fourth digit (our ring-finger) of 

 the hand to the anterior surface of the thigh and leg. According to some 

 authorities, it was the fifth digit (our "little" finger) that became the " big" 

 membrane -supporting finger in the pterodactyl. The tail, long in some 

 pterodactyls but short in others, was inclosed in another membrane which 

 connected the inner surfaces of the thighs and legs. 



At first sight, the wings of pterodactyls would seem to resemble those of 

 bats, but this resemblance diminishes on closer examination. In the bat, 

 not only is the little finger of the hand enormously lengthened, to support the 

 membrane attached to its posterior edge and extending back to the fore sur- 

 face of the thigh and leg, but also the fingers of the bat's hand correspond- 

 ing to our index- middle and ring-fingers are similarly lengthened, and the 

 spaces between them are webbed with membrane as far as their tips. Only 

 the clawed thumb remains small and free from the membrane and projects 

 forward. Our common, insect-eating bats have an additional membrane be- 

 tween the hind legs inclosing the long tail in its middle, but the East Indian 

 fruit-bats, some of which are as large as half-grown kittens, are tailless, and 

 the membrane between the thighs and legs is very poorly developed. 



Flight reaches its most perfect expression in birds, and some of these are 

 the largest flying animals now found upon the globe. All birds' wings — 

 from the huge soaring wings of the condor to the little, rapidly vibrating 

 wings of the humming-bird — are built on the very same plan, — a plan, how- 

 ever, that differs very much from the wing -plan of bats and pterodactyls. 

 In existing birds, that portion of the fore-limb which corresponds to our 

 hand has only three fingers, corresponding to our thumb, index- and middle 

 finger, or, according to some zoologists, to our index-, middle and ring-finger. 

 The thumb is short and unimportant, whereas the index- and middle fingers 

 are united and considerably elongated. To the hind margin of this peculiar 



