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MAMMALIA. 



If the vertebral column terminates in a tail, which it 

 does in most of the genera, though not in them all, the 

 mesial plane also divides that to its very extremity. 

 In walking animals, when on level ground the mesial 

 plane is perpendicular to the horizon ; and it is in 

 this plane that the progressive motion of the walk- 

 ing and the swimming mammalia is performed. If 

 the foot is only a walking foot, and cannot lay hold 

 or be brought to the mouth of the animal, or perform 

 some other peculiar function, then the foot has com- 

 paratively little motion but that of being extended 

 forward or drawn back in this mesial plane. 

 Thus it is of importance to attend to this plane in 

 the animal structure, because it presents us with the 

 simplest view which we can have of the mechanical 

 action of an animal. When we say " mechanical," we 

 must not be understood as having the least intention 

 to revive the old error of supposing that animals are 

 machines ; though the moving of any organ of an 

 animal, and the progressive motion of an animal 

 itself, are, when considered simply in themselves, 

 purely mechanical ; and we leave the mechanism of 

 the matter behind us, and come to the essence of the 

 animal, when we consider the impulse by which the 

 organ or the whole animal is put in motion. With 

 this explanation, the words " animal mechanics" may 

 be used with the strictest propriety in describing both 

 the general structures of animals and their adaptation 

 to general habits, and the particular structures of 

 organs and their adaptation to specific uses in the 

 economy of the animal. In doing this, while we 

 leave the subject of animal life the unseen spring 

 which puts the machine in motion out of the ques- 

 tion, the structure of the mammalia becomes one of 

 the best studies that we have in practical mechanics ; 

 and though it is impossible to find out the precise 

 use of every little peculiarity in the forms of the parts 

 of animals, yet we always find, that when the animal 

 is properly grown and in vigorous health, it exhibits 

 a perfect model of mechanical economy. Every part 

 of it is strong enough for the office it has to perform, 

 and heavy enough for possessing the requisite sta- 

 bility in the performance of it, and this, not only in 

 the ordinary circumstances of the animal's life, but 

 also under a very considerable extent of contingency, 

 so as to make it bear up against those changes in 

 nature which appear necessary for the welfare of the 

 whole system. But, at the same time, no strength is 

 wasted, and no member is overloaded. The active 

 principle of the animal has, therefore, in the organi- 

 sation, the most extraordinary machine which we can 

 imagine ; and this machine is self-supported and self- 

 moving, grows from an embryo by its own vital 

 energy, and is capable of reproduction. Keeping 

 out of view, in the mean time, the vital action of the 

 animal, and also those instincts or impulses which 

 stimulate it to the various external actions which it per- 

 forms, we may say that the mechanical action of an 

 animal in general consists in moving the weight of 

 its own body, or of some part of it. Many animals, no 

 doubt, do carry other things besides their own weight, 

 in a state of nature as well as in a domestic state, but 

 the natural cases of this are so few, and the fact itself 

 blends so naturally with the consideration of the 

 animal moving its own body or the parts of its own 

 body, that this general action may be held as includ- 

 ing the particular one. 



Although, as we have already said, it is difficult, 

 if not impossible, to form a general notion of the struc- 



ture of the mammalia by referring to examples ; yet 

 when we consider that the moving of its own weight, 

 or of itself and some supplemental weight, is the prin- 

 cipal action of all the mammalia, and, with the excep- 

 tion of those which have flying membranes or swim- 

 ming paws, all mammalia take their motions from the 

 earth, or from some other solid, as a fulcrum of resistance, 

 we have one step toward the adaptation of the whole 

 structure of the animal to the performance of this action. 



The motion of the animal must, of course, be either 

 slower or quicker, taken in the general habit, which 

 may in all cases be said to consist in seeking food, 

 though there are many animals which move slowly 

 when feeding, but which are yet capable of very swift 

 and long continued motions ; but when even this is 

 the case, the animal has always some second purpose 

 which is necessary for its subsistence or its safety. 

 In many cases, the food of the mammalia, and es- 

 pecially that of those species which congregate in large 

 herds, is seasonal in many places, and although mam- 

 malia cannot migrate over long distances and cross 

 seas, as the birds can which make their passage 

 through the air, yet there are many parts of the world 

 where wild mammalia are under the necessity of mak- 

 ing pretty long seasonal journeys, as is the case with 

 the antelopes and other animals on the borders of the 

 deserts, and, generally speaking, in all countries 

 which, from periodical rains, are very fertile at one 

 season of the year, but burned up with drought at 

 another : all animals which are placid in nature are 

 swift-footed, though when browsing on a good pasture 

 their motions are comparatively slow. 



Another necessity which animals of the same kind 

 have for swift motion is, that they may be able to 

 escape from their enemies. Many of them, when 

 pushed to extremities, can defend themselves with 

 great resolution, but still the general means of safety 

 with them is flight ; and the predatory mammalia, 

 which are set over them in a state of nature to regu- 

 late their numbers, are all liers in wait, and not fol- 

 lowers of their prey by swiftness of foot. The addi- 

 tional powers of motion with which mammalia are 

 endowed for the purposes of migration and safety, 

 are characters of the particular race however, and 

 not of the general structure, so that we may consider 

 slow motion and quick motion in the natural and 

 usual mode of rinding their food as two limits between 

 which the simple and general motion of all mamma- 

 lia may be considered to lie. 



It must not, however, be supposed that the quick- 

 moving animal performs more motion upon the whole 

 than the slow-moving one ; for it will generally be 

 found that the continuance of the motion is inversely 

 as the rapidity of it. The beast of prey, the lion or 

 the tiger for instance, springs at once on its victim 

 with the swiftness of an arrow, and it feeds ravenously, 

 so that in a very short time its hunger is satisfied, and 

 it lies down and dozes in a state of perfect inaction, 

 until it is again aroused by hunger. The grazing 

 animal, on the other hand, when it is on a bare pas- 

 ture, grazes all day long, and proceeds constantly 

 onward with a comparatively slow motion. 



We might expect some difference, not merely in 

 the structures but in the textures of animals, which 

 vary thus much in their most general action, and we 

 accordingly find that such is the case. The bones of 

 the animal which makes the violent momentary rush 

 upon its prey, and then reposes, are harder and 

 heavier than those of the slow-moving animal. They 



