Let us examine this question at somewhat greater length. From the age of Aristotle to the present time, zoologists have arranged the almost countless tribes of animals into a number of groups or divisions, more or less related in general habits and structure. All the classifications which have been hitherto proposed may be referred to one or the other of two general principles. Either the colour of the blood (Aristotle), the conformation of the heart (Hunter), the arrangement of the nervous system (Owen), or some other peculiarity of _internal_ structure, has been selected as the basis of arrangement; or the animal creation has been grouped according to certain definite circumstances connected with outward form and configuration. As regards their internal organisation, the variations in the structure and arrangement of the nervous system affords by far the best means of classifying animals, and the whole animal world has accordingly been divided into five primary divisions. We have already endeavoured to prove that the manifestations of mind are only made known to us through the medium of corporeal organs, whatever may be the nature of its essence. By the state of the nervous system volition is modified or controlled, the operation of the senses accelerated or retarded, the muscular movements rendered feeble and uncertain, or strong, vigorous, and energetic; in fine, the chief manifestations of life, thought, volition, and independent motion, are all regulated by the condition and structure of the central nervine masses. APPENDAGES TO THE TRUNK A KEY TO THE ENTIRE ORGANISATION AND HABITS OF ANIMALS. The appendages to the trunk, like the arrangement of the nervous system, furnish also a leading peculiarity of structure, and they may be selected as the basis of a classification founded upon outward form. For the most part, destined either for the purposes of locomotion, or to assist in the procuring of food, they must of necessity be in perfect accordance with the nervous and nutritive systems. Hence the capacities, powers, and general habits of an animal, must be in correspondence with this part of its organisation; so that when we are acquainted with the form and arrangement of the extremities of an animal, a key is thereby afforded to its entire organisation. Both its external configuration, its mode of life, and its internal structure, must harmonise, and be in accordance with the arrangement of the extremities. Its breathing and vascular apparatus, its nutritive and generative systems, are thus all indicated to us; and from a knowledge of one particular we may safely and surely predicate the nature of the rest.