The exceptional temperature of the Flying Fish has led to a vague impression that physical activity and its effect upon the amount of blood which vigour of movement circulates, are more important in raising an animal's temperature than possession of the circulatory organs commonly associated with hot blood, which drive the blood in distinct courses through the body and breathing organs. Yet the kind of heart which is always associated with vital structures such as Pterodactyles are inferred to have possessed from the brain mould and the pneumatic foramina in the bones, is the four-chambered heart of the bird and the mammal. Considering these organs alone--of which the fossil bones yield evidence--we might anticipate, by the law of known association of structures, that nothing distinctly reptilian existed in the other soft part of the vital organisation, because there is no evidence in favour of or against such a possibility. CHAPTER VIII THE PLAN OF THE SKELETON While these animals are incontestably nearer to birds than to any other animals in their plan of organisation, thus far no proof has been found that they are birds, or can be included in the same division of vertebrate life with feathered animals. It is one of the oldest and soundest teachings of Linnæus that a bird is known by its feathers; and the record is a blank as to any covering to the skin in Pterodactyles. There is the strongest probability against feathers having existed such as are known in the Archæopteryx, because every Solenhofen Ornithosaur appears to have the body devoid of visible or preservable covering, while the two birds known from the Solenhofen Slate deposit are well clothed with feathers in perfect preservation. We turn from the skin to the skeleton. The plan on which the skeleton is constructed remains as evidence of the animal's place in nature, which is capable of affording demonstration on which absolute reliance would have been placed, if the brain and pneumatic foramina had remained undiscovered. With the entire skeleton before us, it is inconceivable that anatomical science should fail to discover the true nature of the animal to which it belonged, by the method of comparing one animal with another. There is no lack of this kind of evidence of Pterodactyles in the three or four scores of skeletons, and thousands of isolated or associated bones, preserved in the public museums of Europe and America.