So far as the evidence goes, it appears that these fossil flying animals show no substantial differences from birds, either in the mould of the brain or the impress of the breathing organs upon the bones. These approximations to birds of the nervous and respiratory systems, which are beyond question two of the most important of the vital organs of an animal, and distinctive beyond all others of birds, place the naturalist in a singular dilemma. He must elect whether he will trust his interpretation to the soft organs, which among existing animals never vary their type in the great classes of vertebrate animals, and on which the animal is defined as something distinct from its envelope the skeleton and its appendages the limbs, or whether he will ignore them. The answer must choose substantially between belief that the existing order of Nature gives warrant for believing that these vital characteristics which have been discussed might equally coexist with the skeleton of a mammal or a reptile, as with that of a bird, for which there is no particle of evidence in existing life. Or, as an alternative, the fact must be accepted that birds only have such vital organs as are here found, and therefore the skeleton, that may be associated with them, cannot affect the reference of the type to the same division of the animal kingdom as birds. The decision need not be made without further consideration. But brain and breathing organs of the avian type are structures of a different order of stability in most animals from the bones, which vary to a remarkable extent in almost every ordinal group of animals. TEMPERATURE OF THE BLOOD The organs of circulation and digestion are necessarily unknown. There are reasons why the blood may have been hot, such as the evidences from the wings of exceptional activity; though the temperature depends more upon the amount of blood in the body than upon the apparatus by which it is distributed. We speak of a Crocodile as cold-blooded, yet it is an animal with a four-chambered heart not incomparable with that of a bird. On the other hand, the Tunny, a sort of giant Mackerel, is a fish with a three-chambered heart, only breathing the air dissolved in water, which has blood as warm as a mammal, its temperature being compared to that of a pig. Several fishes have blood as warm as that of Manis, the scaly ant-eater; and many birds have hotter blood than mammals. The term "hot-blooded," as distinct from "cold-blooded," applied to animals, is relative to the arbitrary human standard of experience, and expresses no more than the circumstance that mammals and birds are warmer animals than reptiles and fishes.