94 



FIRST CLASS OF THE VERTEBRATED ANIMALS. 



portion than those of the Kangaroo. As the soles of their feet are very small, the 

 metatarsi elevated, with the tail very long and hairy at its extremity, we perceive a 

 striking difference in their mode of progression from that of the Kangaroos. At 

 each leap the Jerboas fall upon their small fore feet, but they raise themselves very 

 quickly by means of a long and heavy tail, serving as a counterpoise, and prevent- 

 ing the body from falling backwards, which accident happens whenever the tail has 

 been lost. 



Among the Rodentia, and especially with the Hares, where this disproportion be- 

 tween the hinder and fore feet is not excessive, the animals usually run at a full gal- 

 lop ; but they avoid descending the declivities of the mountains at this pace, as the 

 forcible action of their hinder legs, aided by the inequality of the ground, would 

 occasion them to perform a somerset. In ascending, they possess, on the contrary, a 

 considerable advantage over an ordinary quadruped, for the gradual rising of the 

 ground counteracts the inequality in the length of the legs, and brings their body 

 more nearly into the horizontal position. 



In the Martens, the Pole Cats, and the Ferrets, which have their feet small, and 

 placed very far apart, the advance is always effected by a full gallop, the back being 

 curved into an arch, in order to bring the two extremities nearer to each other, and 

 to prevent the abdomen from trailing on the ground. 



Among the terrestrial Mammalia, a great many species are obliged to seek their 

 food among the branches of trees. The greater number of these have their thumbs 

 opposable to the other fingers, either on the four feet, as in the Apes and Makis, or 

 on the hinder feet alone, as in several Marsupialia. This arrangement gives to 

 tliese animals the power of seizing the branches with facility, and of applying the 

 entire palm of the hand to the inequalities of surfaces. Other Mammalia, being de- 

 prived of moveable fingers, make use of their long nails as hooks, which fulfil the 

 same purpose, although rather in an imperfect manner, such as we find in the Sloths 

 and Ant-eaters. 



Finally, among the climbing species we find many which make use of their tail 

 as a fifth limb, on account of the surprising flexibility which it possesses, as well as 

 its power of rolling itself round the branches, and even of picking up very small and 

 slender bodies. This tail, termed prehensile (^Cauda prehejisilis), belongs more 

 especially to some American Apes, to the Opossums, the Phalangers of the islands 

 in the Indian Archipelago, to the Brazilian Porcupine {Hijstrix prehensilis)^ and a 

 few others. 



All those Mammalia, which, like Man and the Apes, have the anterior extremities 

 supplied with hands, possess at the same time complete clavicles, and one hand alone 

 is sufficient to seize the food and bring it to the mouth. It is different with all the 

 other animals destitute of thumbs, and whose fingers, being moveable all together, 

 and in one direction, are armed with crooked nails. In these animals, such as the 

 Squirrels and Rats, the assistance of both hands is necessary to hold an object. 

 Hence, while feeding, they are in the habit of sitting down, which enables them to 

 use both of the anterior extremities with freedom. Those Carnassiers which have 

 rudimentary clavicles, and whose paws are well furnished with claws, are obliged to 

 hold their prey between the two fore paws, and tear it with their teeth. The Cats 

 alone ai*e able to carry with one paw to their mouth a very small portion of food by 

 means of their sharp claws. 



Some Mammalia dig in the earth with great facility, and among them the ex- 

 tremities are specially adapted for this purpose. The nails of the fore feet among 

 the digging Mammalia are commonly very strong and short, so that these animals 

 are rarely swift runners, excepting perhaps the Jerboas, which hollow out their dw^ell- 

 ings in the loose sands of the steppes. The Mole, the Scalops, and the Chryso- 

 chloris, are especially remarkable for the strength of their hands, where all the fin- 

 gers are united together, and form a kind of shovel or spade, very well adapted for 

 removing the earth. In these animals the abdomen trails upon the ground. 



In general, we find that all the anomalous points in the organization of these ani- 

 mals adapted for digging, may be reduced to two particulars — to modifications in the 

 anterior extremities, and proportional differences in the organs of sense. The first 

 traces of these anomalies may be fou^id in such Mammalia as burrow in the earth to 

 shelter themselves and their young ones ; but they are most clearly seen in those 

 animals which scarcely ever come from beneath tlie surface of the ground, but are 

 continually in the habit of seeking a subterranean subsistence, whether vegetable or 

 animal. 



The first and most striking examples may be observed among the insectivorous 

 Carnassiers, such as the Mole, the Chrysochloris, and a few neighbouring genera. 

 Among these animals, we find that the anterior limbs are proportionably much more 

 developed than the hinder, which are very slender. The thorax forming the point 

 of insertion for the muscles of the shoulder, acquires a peculiar solidity by the speedy 

 ossification of the cartilages of the ribs, the pieces of the sternum, and by the exist- 

 ence of a kind of breastwork or medial crest. The clavicle being very strong and 

 solid, projects much in front. The shoulder-blade is very broad, with the acromion 

 and coiacoid processes distinctly pronounced, and indicating the presence of muscles 

 of a proportional size. The shoulder loses much of its usual length to acquire a 

 breadth so considerable, as almost to make it square. The two bones of the fore 

 arm, though less strong, are yet tolerably so ; and, finally, the hand, which is shaped 

 like a shovel, is very short and broad, terminated by five very thick and cutting 

 nails, and is still further augmented by a sharp bone as a supernumerary, which oc- 

 cupies the cubital margin. It may be added to this description, that the entire 

 limb, which bears, at its insertipn, upon the lateral parts of the neck, is turned 

 beneath and behind in such a way, that the hand acts constantly in those direc- 

 tions. 



There is another modification in these animals, which properly belongs to the or- 

 gans of motion. The nose or muzzle is modified to be an instrument fit for excavat- 

 ing or digging, at the same time that it serves as a finder through the dehcacy of its 

 olfactory organ ; hence result the ossification of the cartilage separating the nostrils, 

 the great development of the muscles of the nose, and, finally, the great force of 

 the extensor muscles of the head. 



In respect to the organs of sense, we may readily expect that the hearing will be 



developed to an extent proportionably greater than in the other Mammalia, by means 

 of a larger drum, a very short auditive canal, a very wide orifice, and no kind of 

 concha. On the contrary, the sense of sight continually diminishes, until it ends by 

 scarcely existing except in a rudimentary form, as is found in many of the Rodentia, 

 and especially in the Zemni (Spalax tj/phlus). In those species where the anterior 

 limbs are a little less adapted for burrowing, the eyes disappear almost wholly, and 

 the organ of hearing appears, on the contrary, to develop itself in an inverse pro- 

 portion to the former sense. 



In respect to the Mammalia capable of flight, these are of two kinds. Some have 

 simply the skin of the sides extended behind the fore feet and before the hinder, as 

 we see in the Flying Cats (Galeopithecus), the Squirrels, and the Flying Squirrels 

 (Pteromys^, The effect of these expansions of the skin merely consists in prevent- 

 ing the body, when darting from a very elevated situation, the top of a tree for ex- 

 ample, from falling too heavily, and hence their function confines itself to that of a 

 parachute. Other Mammalia, on the contrary, do really fly, that is to say, they can 

 elevate themselves by means of the movements of their anterior extremities, which 

 are prodigiously developed and furnished with long fingers, united together by ex- 

 pansions of a very fine skin ; these are the Bats. 



The first tendency towards the anomalous organization for flight is seen in those 

 Mammalia which are disposed to seek their food in trees, and hence are constituted 

 so as to climb with facility. In fact, there is no Maramiferous animal, before sus- 

 taining itself in the air, which is not obliged to elevate itself to a height more or less 

 considerable. The latter movement may be effected by a mere digital compression 

 of the fore and hind feet, as we see in the Apes ; or by embracing the tree, as in the 

 Bear ; or, finally, by means of hooks, such as we see in the Cats, the Sloths, and 

 even in the Squirrels. The first traces of that disposition by which these animals 

 are enabled to leave their elevated position, and maintain themselves wholly in the 

 air, is found in some Rodentia nearly allied to the Squirrels, being the Pteromys or 

 Flying Squirrels, which have on their flanks a fold of skin, as already described, and 

 likewise in the Phalangista or Flying Opossums. A still more perfect state of this 

 organization is found in some species of Carnassiers nearly allied to the Apes, such 

 as the Galeopithecus. In these animals, the fore extremity is considerably elongated. 

 The fore arm has acquired that peculiar ginglymoidal arrangement, which has been 

 assigned to the Cetacea for a different purpose ; and, as may be easily supposed, the 

 principal moving, and especially the pectoral, muscles are very largely developed. 

 On the other hand, the hinder limbs have diminished very sensibly, and also the tail, 

 which is almost wanting, in order to throw the centre of gravity of the body within 

 the axis of the fore extremities. The most perfect form of this anomaly is, how- 

 ever, seen in the Bats. 



The aquatic Mammalia are of two kinds ; some frequent the margins of lakes, 

 rivers, or streams, into which they plunge occasionally to seize their prey, or to find 

 the aquatic plants on which they feed. Their extremities are either wholly webbed, 

 as in the palmated feet of the Ornithorhynchus, in which the expansion of the skin is 

 that of the palm or sole of the foot, and not merely of the fingers, as we find in the 

 Otters and Cabiais. These animals have webs upon all the four feet, being different 

 from the Beavers and Cheironectes, whose hinder extremities alone are webbed, 

 while the skin extending between them is entire. In the Desmans, the feet are only 

 semi-palmated, and in the Hydromys, they are palmated for two- thirds of their 

 length. 



The Ondatra, whose mode of life is so analogous to that of the Beaver, has its 

 hinder feet fitted for swimming ; but instead of having the feet united by a mem- 

 brane, each of them is bordered on the right and left by a row of elongated, stiff, 

 and serrated hairs, which cross each other by the points witb those of the adjoining 

 fingers, thus forming a surface capable of offering sufficient resistance to the 

 water. 



Among the marine Mammalia, the Seals and Morses deserve to be noticed in 

 the first place, because they are supplied with all the four extremities, of which the 

 anterior have the fingers united, and armed with claws. In some species of Seals, 

 being the Otaries of Peron, the skin on the tip of each finger is prolonged into a 

 long and narrow strap, forming a band. The hinder feet, placed entirely at the ex- 

 tremity of the body, also have apparent fingers, but these are united together by the 

 skin. Others, such as the Lamantins and Dugongs, are destitute of the hinder ex- 

 tremities ; but all the fingers of their limbs are invested by a thick skin, on which 

 vestiges of nails are to be found. Finally, the proper Cetacea, which also have the 

 hinder extremities wanting, depart still more widely from the quadrupeds, as we 

 here fail to discover the slightest trace of nails. These differences in the organiza- 

 tion of the extremities occasion corresponding variations in their mode of life. The 

 Seals are very agile when in the water, and execute a number of evolutions and 

 movements in consequence also of the extreme flexibility of their vertebral column ; 

 but they walk with much apparent difficulty on the land. The Lamantins, Du- 

 gongs, and Stelleres (JRytina), have still greater difficulty in leaving the water, and 

 remain like inert maspes whenever they chance to run aground upon a bank. The proper 

 Cetacea do not voluntarily leave the sea, where they swim with a prodigious velo- 

 city, by means of the movements of their tail and fins. In consequence of the 

 former being flattened from above downwards, and not compressed from right to left, 

 this organ moves chiefly in the vortical direction, instead of horizontally, as we find 

 in the greater part of the Fishes. 



On compai'ing all aquatic Mammalia with each other, and with the remaining ani- 

 mals of their class, we find that the organic modifications which have been experienced 

 by those MammaUa destined to reside more or less in the water, consist essentially 

 in the following particulars : — The Hairs which, in the amphibious animals, are ob- 

 served to be short and exceedingly numerous, terminate in the Cetacea by forming 

 an agglutinated or universal envelope, as we see in the Lamantins and Whales. The 

 external Ear diminishes in size, until it ends, in certain Cetacea, by disappearing al- 

 most wholly, so that it is scarcely possible to discover any vestige of the concha. 

 The crystalline lens of the Eye is observed gradually to become more convex, and to 

 approach insensibly towards the spherical form, which it acquires most completely in 

 species residing constantly in the water. We may further remark the absence of 



