140 



M A M M A L I A. 



whether that surface is level, or an ascent, or is de- 

 scending'. In the case of ascent or descent, there is of 

 course a degree of inclination, beyond which no 

 walking animal can execute its motion, though there 

 are great differences in this respect arising from the 

 structure of the organs of locomotion. Thus, for 

 instance, if the fore legs of the animal are short in 

 proportion to the hind ones, as in the hare for 

 instance, the long hind legs project the body very 

 readily and conveniently up hill in running; whereas, 

 if an animal so formed attempts to proceed rapidly 

 down hill, it is in continual danger of tumbling heels 

 over head. Hence, coursed hares always endeavour 

 to take the hill of the dogs, if there is one accessible 

 to them ; and if there is a sufficient length of hill 

 they are safe, if not too much exhausted before ; but 

 if they are compelled to run down hill, they very 

 speedily become the prey of their pursuers. 



These two sub-classes, or orders of motion origi- 

 nating from solid fulcra, admit of far more diversity in 

 the mode of performance, and require a much greater 

 variety of organs, than motions in which air or water 

 is the fulcrum. The reason of this is very obvious. 

 Air and water are very uniform in their consistency. 

 There is very little difference in the density of water 

 at different depths, because water cannot be much 

 compressed even by very heavy pressure. The 

 movement of an animal through water, therefore, 

 differs scarcely any thing in species, whether the 

 depth be greater or less, and therefore the very same 

 organisation serves equally for motion through it, 

 whether the animal is near the surface or farther 

 down. Atmospheric air differs from water in being > 

 more compressible, and consequently it varies more 

 in density at different elevations above the mean ! 

 level of the earth's surface. But still there is very ' 

 little difference to a flying animal arising from this ; ! 

 for the resistance of the air to the progressive motion j 

 of the animal diminishes at probably a greater, and , 

 certainly as great a rate as its density. Something 

 depends on the velocity with which the animal flies 

 against the air ; and the theoretical view of the 

 matter being that the resistance is directly as the 

 density of the medium and the square of the velocity 

 jointly, a comparison might be instituted. This theo- 

 retical view never can be made, however, to agree 

 with experience, or rather experience does not agree 

 with it ; for though it is nearly correct for moderate 

 rates, yet, when we come to great velocities against 

 the same resisting medium, we do not find the actual 

 resistance to increase nearly so fast as the square of 

 the velocity. For the purposes of natural history it 

 is not necessary to go into those niceties ; and we 

 may assume that a flying animal gets along with 

 nearly equal exertion in proportion to the rate of its 

 .motion at whatever height it may happen to be in 

 the atmosphere. Or if there is a deviation from this, 

 the circumstances of the case would lead us to con- 

 clude that flight is more easy on a high course than 

 on a low one, for we invariably find that even birds 

 of heavy flight rise high in the sky when they are to 

 perform long aerial journeys. 



Thus in all the flying mammalia, and also in all 

 the swimming ones, there is little diversity required j 

 in the organs of motion ; and when we have described ! 

 those organs in one species, either of the one or the ' 

 other, we are in possession of the knowledge of them j 

 all, excepting in so far as they may be modified by a 

 participation in other habits, in addition to the flying 



or the swimming. There u another circumstance 

 which further simplifies the structural study of both 

 these descriptions of mammalia ; their extremities are 

 never any thing else than organs of motion, unless 

 they happen also to be weapons of defence, as is the 

 case with the tail of the whale. In no case, however, 

 do they in any way assist the animal in the seizing of 

 its food, they merely conduct it to the locality in 

 which the food is to be found, and thus we can study 

 their system of motion, as a simple system unmixed 

 with any other. 



When, however, we come to the land animals, or 

 those which take their movements from solid fulcra, 

 the case is very different, and this renders their mo- 

 tions and their organs of motion a far more intricate 

 study. There are no doubt some of them which are 

 incapable of making much, if any, use of their organs of 

 motion, except in moving from place to place ; but the 

 number of these among walking animals is compa- 

 ratively limited ; and among climbing animals there 

 are scarcely any. The climbing foot is, from the use 

 to which it is applied, almost necessarily prehensile in 

 some respect or other, either in grasping like a hand, 

 or in holding on by means of claws. When it takes 

 the hand form, however much it may differ from the 

 human hand, which is a universal instrument and not 

 a grasping one, it is of course equally fit for laying 

 hold of the food, as for grasping the branch of a tree 

 or whatever else is the means of support in climbing. 

 An animal which climbs by grasping, always requires 

 to have clavicles, in order to support the shoulder 

 joints in those cross motions which the action of 

 reaching from branch to branch requires ; the same 

 may be said of climbing animals which use mem- 

 branes, or even simply their spread out legs, to pro- 

 tract their fall when they leap ; and so also do 

 animals which climb by grasping the bole of a tree, 

 or other comparatively slender substance between the 

 feet in climbing. But, with the exception of the 

 winged mammalia, and they have a little of it, all 

 mammalia which have a cross motion of the tore 

 legs, have also a motion of them forwards and back- 

 wards. Hence every such animal can bring the fore 

 foot to the month, and when an animal can do this, 

 it always uses that foot more or less in the manage- 

 ment of the food. We see remarkable instances 

 of this in the ape and monkey tribes, some of the 

 former of which are so expert in the use of the hands 

 or fore paws in this way, that they not only gather 

 their food with them, and clear it from refuse if neces- 

 sary, but they make use of the finger in picking their 

 teeth, if any substance adhering to these or sticking 

 between them gives them uneasiness. The squirrels 

 and other climbing animals which do not grasp, but 

 retain their hold or their footing by the toes and 

 claws, have also remarkable power over the anterior 

 extremities ; and they carry their food to the mouth 

 with their paws, and use the paws in preventing it 

 from falling, when it requires considerable action of 

 the jaws to reduce it to the necessary form. 



Even those animals which do not subsist upon 

 fruits which they pull from trees, or upon insects and 

 other things which they draw from the holes of trees, 

 but which are predatory in their climbing, and catch 

 living animals, very generally use the foot, either for 

 grasping the prey by means of crooked claws, or by 

 striking it against the branch, and holding it there, 

 until they can seize it, and if necessary dress it with 

 the mouvh. 



