CHAPTER XVII
FAMILY RELATIONS OF PTERODACTYLES TO ANIMALS WHICH LIVED WITH THEM
Enough has been said of the general structure of Pterodactyles and the chief forms which they assumed while the Secondary rocks were accumulating, to convey a clear idea of their relations to the types of vertebrate animals which still survive on the earth. We may be unable to explain the reasons for their existence, and for their departure from the plan of organisation of Reptiles and Birds. But the evidence has not been exhausted which may elucidate their existence. Sometimes, in problems of this kind, which involve comparison of the details of the skeleton in different animals, it is convenient to imagine the possibility of changes and transitions which are not yet supported by the discovery of fossil remains. If, for example, the Pterodactyle be conceived of as divested of the wing finger, which is its most distinctive character, or that finger is supposed to be replaced by an ordinary digit, like the three-clawed digits of the hand which we have regarded as applied to the ground, where, it may be asked, would the animal type be found which approximates most closely to a Pterodactyle which had been thus modified? There are two possible replies to such a question, suggested by the form of the foot. For the old Bird Archæopteryx has three such clawed digits, but no wing finger. And some Dinosaurs also have the hand with three digits terminating in claws, which are quite comparable to the clawed digits of Pterodactyles.
The truth expressed in the saying that no man by taking thought can add a cubit to his stature is of universal application in the animal world, in relation to the result upon the skeleton of the exercise of a function by the individual. Yet such is the relation in proportions of the different parts of the animal to the work which it performs, so marked is the evidence that growth has extended in direct relation to use of organs and active life, and that structures have become dwarfed from overwork, or have wasted away from disuse—seen throughout all vertebrate animals, that we may fairly attribute to the wing finger some correlated influence upon the proportions of the animal, as a consequence of the dependence of the entire economy upon each of its parts. Therefore if an allied animal did not possess a wing finger, and did not fly, it might not have developed the lightness of bone, or the length of limb which Pterodactyles possess.
The mere expansion of the parachute membrane seen in so-called flying animals, both Mammals and Reptiles, which are devoid of wings, is absolutely without effect in modifying the skeleton. But when in the Bat a wing structure is met with which may be compared to a gigantic extension of the web foot of the so-called Flying Frog, the bones of the fingers and the back of the hand elongate and extend under the stimulus of the function of flight in the same way as the legs elongate in the more active hoofed animals, with the function of running. Therefore it is not improbable that the limbs shared to some extent in growth under stimulus of exercise which developed the wing finger. And if an animal can be found among fossils so far allied as to indicate a possible representative of the race from which these Flying Dragons arose, it might be expected to be at least shorter legged, and possibly more distinctly Reptilian in the bones of the shoulder-girdle which support the muscles used in flight. It may readily be understood that the kinds of life which were most nearly allied to Pterodactyles are likely to have existed upon the earth with them, and that flight was only one of the modes of progression which became developed in relation to their conditions of existence. The principal assemblage of terrestrial animals available for such comparison is the Dinosauria. They may differ from Pterodactyles as widely as the Insectivora among Mammals differ from Bats, but not in a more marked way. Comparisons will show that there are resemblances between the two extinct groups which appeal to both reason and imagination.
Dinosaurs are conveniently divided by characters of the pelvis first into the order Saurischia, which includes the carnivorous Megalosaurus and the Cetiosaurus, with the pelvis on the Reptile plan; and secondly the order Ornithischia, represented by Iguanodon, with the pelvis on the Bird plan. It may be only a coincidence, but nevertheless an interesting one, that the characters of those two great groups of reptiles, which also extend throughout the Secondary rocks, are to some extent paralleled in parts of the skeleton of the two divisions of Pterodactyles. This may be illustrated by reference to the skull, pelvis, hind limb, and the pneumatic condition of the bones.
FIG. 77. COMPARISON OF THE SKULL OF THE DINOSAUR ANCHISAURUS WITH THE ORNITHOSAUR DIMORPHODON
