CREATION BY EVOLUTION
merely reptiles that were especially adapted to aerial navigation. Nature essayed this adaptation twice with the reptiles, one product being the pterodactyls or winged reptiles of the Mesozoic era. The pterodactyls and the birds have much in common, even to certain minute details of pneumatic bones with comparable openings of communication one with another: lightness of skull, with a precocious consolidation of cranial bones, and loss of teeth and their replacement by a beak in later forms. The marked distinctions lie in the mechanism of flight—by wing feathers in the birds and by a bat-like wing membrane in the pterodactyls. The two groups are doubtless derived from the same or a related non-flying ancestry, and much of their similarity is probably due to community of habit and its reaction on the mechanical structure.
The links leading to the pterodactyls are still unknown, for our first fossil record of these creatures are the remains of forms that had already attained sustained flight; of the first steps in that direction we have no direct knowledge. Not all conditions of life, however, are equally susceptible of preservation in the fossil record, and this is particularly true of flying forms, for a prerequisite to fossilization is complete burial either in water-borne or air-borne sediments, such burial as befalls flying forms only in very exceptional circumstances. A single locality, a quarry near Solenhofen, Bavaria, worked commercially for limestone used in the art of lithography, has given us nearly all the flying creatures of Middle Jurassic age that we know. Here are pterodactyls, some of which have the delicate wing membrane preserved in perfect detail; but these are already highly specialized fliers. Here also are the first known birds, feathers and all, and among the whole list of connecting links known to us none is perhaps more satisfying. Three individuals are rep-
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