It is only by means of a minute knowledge of compara tive anatomy that such investigations can be conducted ; and hence geologists, and even naturalists in general, to whom this study is not easy of access, are rarely in a situa tion to make discoveries of this kind, but must . be con tent to depend on the very few observers who may have made this branch their particular business. The more necessary is it for them to take care that they do not place confidence in any but the best authority on these subjects, and to receive with caution the presumed discoveries of those, whose limited experience in this department of na tural history, and insufficient acquaintance with general as well as particular comparative anatomy, must render them Incompetent judges in questions so obscure.
Such, however, are the correspondences and relations in the forms of all such organized bodies, according to the several ends to which they are directed, that to an acute and experienced anatomist there is scarcely one part in the skeleton from which the nature of the others cannot be deduced. In every individual there is a set of peculiar actions and habits of life directed to a particular object or set of objects, to attain which all the parts must possess some general concatenations of forms and of mutual depen dence. Thus, as no one part, in examining detached por tions, can deviate from its fixed form without some corres ponding deviation in all the rest, so, by investigating the varieties of that, all the other parts of the animal may be inferred with considerable safety.
If, for example, the viscera of any animal are adapted for digesting the flesh of living animals, the jaws and teeth are also fitted to devour the prey, the claws to seize and detain it, the limbs to pursue it, and the organs of sense to discern it from a distance. From such general rules there are minor deviations, depending on a variety of causes, such as the nature of the prey, as well as its size and habits; but these only cause the modification of small parts, while the essential arrangements are still followed. It is thus that, while these rules, taken in the widest sense, determine the class, and even the order of any animal, the genera and species are discovered by attending to the va riations which these undergo in some of the accessory and inferior points of structure.
To examine the particulars more minutely; if the jaw is intended for securing objects that offer resistance, it is necessary that its joint should possess a particular form, and that there should be a certain relation between the moving force, the fulcrum, and the resistance. For the
sake of gaining sufficient strength, the temporal muscles must also be powerful; and hence is determined the depth of the depression in which they lie, together with the con vexity and the strength of the zygomatic process. It is necessary, also, that in such animals the muscles of the head should be powerful ; without which they would be unable to carry off their prey. Hence a peculiar form becomes requisite for the vertebra:, and for the back part of the head, where the attachments of these muscles are placed.
Sharp teeth are also required for the purposes of ani mals of this nature, with strong roots firmly fixed; and, in conformity to this necessity, the forms of the parts which are concerned in the motions of the jaws are also deter mined. In a similar manner, as the claws of carnivorous animals are employed for seizing their prey, it is requisite that they should be strong, and that the whole of the toes should have considerable freedom of motion. Hence the forms of the paws, and those of all their parts, are deter mined, as well as the distribution of the muscles and ten dons by which they are to be moved. To proceed a step further ; as it is necessary that the fore arm should possess great facility of moving in various directions, the bones of of which it is composed are limited to a particular class of forms. These, again, necessarily determine the shape of the bones of the humerus ; and as the scapula must neces sarily possess great strength in these cases, as well as con siderable muscular powers, there also results a necessary limit of forms to which these are all reducible.
This train of necessary dependence may be carried on through the hinder limbs, and so on to the trunk and all the other parts of the animal ; and in this way the necessary flexibility, as well as strength, determines the structure of the vertebrae, and of all the other parts of the trunk. In the same manner, too, when we consider what the organs of sense ought to be in such animals, to enable them to carry on the business of their life, we are prepared to judge of the shapes of the bones which are required for the eyes, the nose, and the ears.