Classifications represent more or less faithfully the gradational increase in the sum of the characters of an organism, as well as the increase in importance that those character severally attain. Thus gathering, so far as may be, from the chaos of individuals, _a common plan of structures_ on which the genus, order, or class is moulded from a less specialized group of organs. The fundamental structures of a vertebrate animal, so far as their persistent importance can be measured, are, those connected with I. Perpetuating the race. II. Construction of the brain. III. Circulation and oxidation of the blood. IV. Locomotion, i.e. skeleton, muscles, &c. And these characteristics are for the most part so interlinked, that it becomes difficult to assign to one order of animals a relative superiority over another order; since when a single set of organs is prominently developed in one group it often happens that another set of organs has a like pre-eminence in an allied group. Thus among reptiles it might be considered that _Crocodiles_ have the best hearts, and _Turtles_ the best lungs. And since these structures in their functions severally modify and determine the use of other structures, the meaning that terms like Crocodilian and Chelonian really have is that they represent the aspect of Reptilian organization when seen through the specialization of respiration, or circulation of the blood. The soft parts thus determining the nutrition and function of the muscles and skeleton, anatomists in examining the bones of extinct animals are accustomed to reverse the order of their inferences, and infer from modifications of the skeleton what had been the characters of the soft and more vital structures. On the presumed accuracy of this method of research rest many results of Comparative Anatomy. But since the shapes of bones are determined by the muscles as well as by inheritance, it is always to be remembered that a similar form of bone may obtain in different orders or classes of animals, as the result of a similar function in a special region of the body. Such resemblances are familiar to anatomists. Hence much caution is required from the Palæontologist to distinguish between the characteristics of a group, and the extent to which they may be modified by function. This distinction is the first principle of classification. But it is always difficult to estimate the importance of characters in fragments of bones or parts of skeletons, and the difficulty is increased by the fact that if what appears to be but a functional modification should pervade all the species, it becomes a characteristic of the group, and its power of modifying the other organs in a peculiar way has to be considered.