If, therefore, the cup-and-ball articulation of vertebræ in Ornithosauria has any significance as a mark of affinity to Reptiles, it could only be in approximation to those living Reptiles which possess the same character, and would have it on the hypothesis that both have preserved the structure by descent from an earlier type of animal. This hypothesis is negatived by the fact that the cup-and-ball articulation is unknown in the older fossil Reptiles. Although the articulation for the lower jaw with the skull in Ornithosaurs is only to be paralleled among Reptiles, the structure is adapted to a brain case which is practically indistinguishable from that of a Bird, except for the postorbital arch. The hypothesis of descent, therefore, becomes impossible, in any intelligible form, in explanation of distinctive character of the skeleton. The hypothesis of elimination may also seem to be insufficient, unless the potential capacity for new development be recognised as concurrent, and as capable of modifying each region of the skeleton, or hard parts of the animal, in the same way that the soft organs may be modified. From which we infer that all structures, which distinguish the several grades of organisation in modern classifications, soft parts and hard parts alike, may come into existence together, in so far as they are compatible with each other, in any class or ordinal division of animals. Although the young Mammal passes through a stage of growth in which the brain may be said to be Reptilian, there is no good ground for inferring that Mammal or Bird type of skeleton was developed later in time than that of Reptiles. The various types of Fishes have the brains in general so similar to those of Reptiles that it is more intelligible for all the vertebrate forms of brain to have differentiated at the same time, under the law of elimination of characters, than that there should be any other bond of union between the classes of animals. If we ask what started the Ornithosauria into existence, and created the plan of construction of that animal type, I think science is justified in boldly affirming that the initial cause can only be sought under the development of patagial membranes, such as have been seen in various animals ministering to flight. Such membranes, in an animal which was potentially a Bird in its vital organs, have owed development to the absence of quill feathers. Thus the wing membrane may be the cause for the chief differences of the skeleton by which Ornithosaurs are separated from Birds, for the stretch of wing in one case is made by the skin attached to the bones, and in the other case by feathers on the skin so attached as to necessitate that the wing bones have different proportions from Ornithosaurs.