M A M M A L I A. 



159 



As the vegetable food of all the ground feeders of 

 nocturnal habits and with digging feet is of that kind 

 which we have described as being considered as food 

 intermediate between animal matter and vegetable 

 matter, it is natural to suppose that all these animals 

 should be more or less omnivorous, or capable of 

 subsisting on the flesh of vertebrated animals, on that 

 of invertebrated [ones, or on vegetables ; though it 

 does not follow that they could, at least in many of 

 the species, subsist upon green leaves or vegetable 

 fibres ; because their cheek teeth are not adapted to 

 the preparation, or their alimentary apparatus to the 

 digesting of these. We find that in this again we are 

 borne out by observation in every case where our 

 ! observation can be applied. Rats, and even mice, 

 / are great consumers of insects ; and in some places 

 where the lower apartments of houses are infested 

 with cockroaches, the inhabitants find it necessary to 

 encourage mice for the purpose of getting rid of 

 these more destructive and far more loathsome crea- 

 j tures. The pulpy and farinaceous matter, upon which 



!all the ground mammalia to which we have alluded 

 as vegetable feeders subsist, must be considered 

 as vegetable fat, that is, as holding the same place in 

 ' the vegetable kingdom as fat does in the animal : 

 that is to say, neither of them is a perfectly organised 

 substance, but each is an assimilated matter, the one 

 vegetable and the other animal, ready for the supply 

 of materials of organisation, when these are either too 

 imperfectly developed, or in a state of too much ex- 

 haustion for being capable of performing the necessary 

 labour of assimilation for themselves. True to this corre- 

 spondence between their vegetable and their animal 

 food, those mammalia prefer fatty or partially organised 

 substances, or substances not organised at all, to those 

 of which the organisation is perfect. Cheese and 

 bacon are proverbially the staple baits for a rat-trap ; 

 and mice attack these substances in the larder in pre- 

 ference to joints of the finest meat, nor is it probable, 

 except in cases of extreme hunger, that any of these 

 animals will gnaw the tough fibres of a well-organised 

 muscle. 



The structure of the digging foot with which those 

 ground mammalia, to what order soever they belong 

 in the system, are provided, requires no lengthened 

 detail, and indeed no mention. To give good effect 

 to a digging foot it must not be too long, for that 

 would render it less effective, because the muscles 

 having to act against a greater length of lever, would 

 have their power proportionally diminished. On this 

 principle the short fore-leg of the jerboa is a far better 

 <liirgiiig leg than the hind leg would be, which is six 

 times the length, although it is just as inferior a 

 walking one, and that from the very cause which 

 makes it so superior for digging. 



It may be worth while to notice the mechanical 

 principle upon which this difference depends, and the 

 adaptation of that principle to the scene of action in 

 both cases. In the foot for rapid or powerful motion 

 in the air, originating from a solid base, or point of 

 resistance in the earth, the grand desideratum is velo- 

 city. The resistance of the air to the passage of any 

 of the average size of the mammalia is but small, and 

 from every kind of experimental knowledge which we 

 have of substances projected from a fixed point on the 

 earth into the air, we have reason to conclude that 

 the range depends wholly upon the velocity. But 

 the way to obtain velocity in the jointed limbs of an 

 animal is to have the origin of the muscle which is 



to move a distal bone, placed as far as possible, 

 proximal of the joint, upon a long bone. This por- 

 tion, taken to the centre of articulation in (he joint, 

 may be considered as one arm of the lever ; and at 

 any supposed flexure of the joint, the other arm of 

 the lever, which represents the power of the muscle, 

 is the perpendicular from the centre of motion at this 

 point ; and in proportion as this is less than the 

 other, its power would be reduced in moving the 

 distal bone. But because the muscle is in itself ex- 

 actly the same in what ratio soever it is apportioned 

 to the two bones, its power must be the same, upon 

 the obvious principle that a quantity remains the 

 same in its total amount into whatever ratio of parts 

 it is divided. In consequence of this, if the power is 

 reduced to one-tenth, the velocity will be increased 

 ten-fold, and so on in the case of every other propor- 

 tion. This is the real reason why all fleet animals 

 have long legs, but we deferred mentioning it until we 

 could contrast it with the reason for animals having 

 short legs. 



In the feet of a digging animal, it is strength and 

 not velocity which is necessary ; and therefore the 

 bones require to be short, so that the lever power of 

 the muscle may be a maximum ; and in proportion as 

 the length, from the origin upon the one bone to the 

 joint, becomes less in proportion to the distance from 

 the joint to the insertion on the other bone, the power 

 of an equal muscle is increased, and the velocity with 

 which it can move the bone diminished in the same 

 ratio. It follows from this, that an animal, the whole 

 of whose feet are adapted for digging, must of neces- 

 sity be a slow walker, and this is the reason why, 

 when the peculiar nature of the posture of an animal 

 of this kind renders a quick motion along the surface 

 necessary, the hind legs are produced to so dispro- 

 portionate a length compared with the fore ones. 

 This structure would not suit an animal which had to 

 seek its food by perpetual burrowing under the 

 ground ; because such an animal requires a powerful 

 action of the hind legs as well as the fore ones, and 

 does not require a greater capacity of swift motion 

 in the one pair than in the other. When, however, 

 the burrowing is only an occasional operation, as 

 that of digging a lodging in the ground, or scraping 

 away the earth to arrive at a bulb or other root, the 

 hind legs admit of that production which is essen- 

 tial for quick motion along the surface of the ground ; 

 and, as has been already observed, such animals must 

 necessarily perform their rapid motions by leaping, 

 and not by running or galloping. In this double 

 adaptation of the animal, there is necessarily some 

 waste of its whole power, just as there is waste of the 

 whole power of a single organ, in all cases where that 

 organ has a double function to perform. The two 

 adaptations of the whole animal are, however, in all 

 cases, as beautifully proportioned to the relative use 

 which it has for the two kinds of action, as those of 

 the single organ are adapted to the use made of its 

 two functions. If one predominates greatly above 

 the other, the adaptation is correspondingly great ; 

 and in proportion as the uses of them approach to an 

 equality the adaptations do the same ; so that, though 

 a loss upon the whole is, from the very nature of the 

 case, inseparable from a double adaptation, there is 

 never more than the average loss in any one instance. 



The same rule applies in the mechanical action of 

 the human body, and there is a good deal of practical 

 instruction to be derived from attending to it in that 



