The most absolute proof that it was a flying animal is the pneumatic character of its bones. This condition was discerned by me in some Pterodactyle bones from the Lias of Franken (Jahrb. für Mineral, 1837, p. 316), and was afterwards established by Owen in the Pterodactyles from the Chalk of England. This structure was previously only known in birds. And the supposition readily follows that in the respiratory process there was some similarity between the Pterodactyle and the Birds. They have the proportions of upper-arm and fore-arm which characterize birds of great flight, the humerus short and the fore-arm long; hence it may be presumed that Pterodactyles could fly well. From the absence and presence of the bony sclerotic ring in the eye, it may be supposed that the Pterodactyles were active in the day-time, while Rhamphorhynchus was nocturnal.
After this statement von Meyer gives a discursive summary, in which his views of the classification of reptiles in general and of Pterodactyles in particular are epitomized. And then goes on to combat the views of people who have departed from his classification and attempted to set up classifications of their own; and cites a number of authors who, labouring at the vertebrata, have endeavoured to find a resting-place in their systems for the Pterodactyle. But the chief thing we learn of von Meyer's own views is, that in 1830 he published a classification of extinct Saurians, dividing them into those with limbs like the larger and heavier land-mammals, those with fin-like limbs, and those with a flying-finger. Which divisions have been widely adopted, though authors have sometimes given them other names than those by which they were first made known.
Von Meyer has freely stated the facts about the Pterodactyle, and draws the conclusion that the animal was a reptile; but how such a conclusion was obtained from such facts is a matter on which his pages are silent. One seems to hear the chirrup of the bird in almost every paragraph. The head is in the main a bird's head; the pectoral girdle and the sternal ribs are those of a bird; and very few are the structures in which some reminder of the bird is not present; and in their bones he discovered the pneumatic characteristic and inferred, for the animals bird-like lungs. How, then, comes it that the Pterodactyle is a reptile? We can only suppose the answer to be, Because if the head and pectoral girdle and other bones had been reptilian it would have been a bird.
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In the views here epitomized it is difficult always to make out the logical foundations of the conclusions arrived at. Sometimes they have no foundations, and sometimes they represent the different aspects in which a truth presents itself to minds differently constituted or differently conversant with the structures of living animals. In now stating my own views I shall avail myself of the example of some previous writers, and attempt to investigate the Pterodactyle as though they had not written. And then, having placed before him all the theories that are known, the reader will be able to choose the theory that pleases him best, if indeed he needs one.
Much of the discrepancy of opinion that exists is probably due to the use of the inductive method of thought for the discovery of fundamental principles in classification. In palæontology, where the types are more generalized than are living forms, it must always be difficult to reason from the known to the unknown. The known is always more or less incomparable with the unknown; and there can be no reason for inferring that the specialities of structure which now accompany specialities in organization would justify us in inferring for the animal, in which the structures formerly were united, the combined organizations of the living animals in which they are now found. On any hypothesis of evolution it would be allowed that the special modifications of a group were attained subsequently to the common plan of the larger group to which it belongs, and are entirely to be attributed to the function which the necessities or organization of the animal caused its structures to subserve. Inductive thought may sometimes discover function from structure, but never makes more than an approximate guess when it endeavours to determine fundamental organization from osseous structures which are not fundamental. And before a naturalist can say, since an animal has for instance a tail like a mammal that in so far it must be affiliated to the mammalia, he must have determined why the mammalian tail has its peculiar characters, and whether it is compatible with any other common plan of organization. And perhaps it might with equal reason be considered reptilian.
Therefore I prefer at firsts instead of reasoning from the details of structure, to adopt the à priori method, and ask, not what the Pterodactyle is like in its several bones, but what common plan it had whereon its hard structures were necessarily moulded. For I imagine, if it can be determined what the nervous and respiratory and circulatory structures of the Pterodactyle were, it becomes a secondary matter to know whether the phalanges are like a lizard's, or the pelvis like that of a mammal. If the animal is asserted to be a mammal, a reptile, or a bird, we ought to be able to adduce evidence that it had the soft parts which are deemed distinctive of the selected class. This no one has done or attempted to do.
Hereafter it will be necessary to describe the Pterodactyle's brain.
There is no organ more distinctive between hot-blooded animals on the one hand, and cold-blooded animals on the other, than the brain. In the cold-blooded groups, or those in which respiration is feeble and circulation imperfect, that is to say, in existing fishes, amphibians, and reptiles, the parts of the brain are arranged one behind another, so that when looked upon from above, a portion called the optic lobes intervenes between the anterior masses called the cerebrum and the posterior mass called the cerebellum. In the hot-blooded groups, or those with an enormous extent of lung-surface for oxidation of the blood and a four-celled heart for its rapid circulation, that is to say, in birds and mammals, the front part of the brain called the cerebrum is immensely developed in proportion to the other parts, and abuts against the cerebellum and more or less completely covers the optic lobes, which in birds are squeezed out to the sides. The Pterodactyle brain is of this latter kind. And it being taken as a postulate that this kind of brain is the product of the organization which produces hot blood, it follows that the Pterodactyle was a hot-blooded animal.
Again, the Pterodactyle has perforations for pneumatic cells in many of the bones.