MAMMALIA. 



187 



one stage of its being ; and as the mammalia pass 

 through only one stage, they cannot be aquatic at one 

 time and terrestrial at another. By living on land or 

 in the water, must be understood as being capable of 

 breathing the free air, or air through the medium of 

 water ; for as no animal can exist without breathing, 

 no animal can be said to live where it cannot breathe. 



The amphibia, though they have their teeth formed 

 according to the general type of the carnivora, can 

 hardly be called carnivorous animals. They are 

 rather piscivorous, for they feed upon fish, not upon 

 the flesh of warm-blooded animals ; and though the 

 muscles of all animals are considered as flesh in a 

 natural history point of view, yet, in ordinary lan- 

 guage, a very wide distinction is made between flesh 

 and fish. 



Still, besides having the same general form of teeth, 

 those amphibia have the general structure of body, 

 and many of them all the sagacity of the laud carni- 

 vora ; though the element in which they find their 

 food requires that their organs of motion, and also the 

 general forms of their bodies, should be different. 

 When an animal, starting from the solid ground in its 

 progressive motion, has to make way only against the 

 resistance of the air, the shape of its body does not 

 require much adaptation to the overcoming of that re- 

 sistance ; and as the resistance is small, much adapt- 

 ation to it would be thrown away. If indeed the ani- 

 mal is to get through the air with great rapidity, then 

 there is a slight adaptation ; but estimating from 

 about the average of animal motion, there may be 

 said to be none. It is very different in the case of 

 water ; and, therefore, when mammalia are sent to 

 seek their food in the waters, the general shapes of 

 their bodies must be adapted to that element. This 

 is effected by an approximation to the typical form of 

 a fish. The feet are short, much enveloped in the 

 skin, webbed between the toes ; and the posterior 

 ones are united with the tail. Their bodies are long 

 in proportion to the diameter ; and the spine is capa- 

 ble of very varied and powerful motion ; so that 

 besides their fore paws, and their compound posterior 

 extremity, the amphibia may be said to swim with the 

 whole body. 



They do not resort to the sea, or other water, 

 merely for the purpose of catching their prey, and 

 come on shore to eat it, as is done by the otters, 

 which, though web-footed, are digitigrade. They eat 

 it in the sea ; but they frequent the shores, and are 

 partial to basking in the sun. They are indeed coast 

 animals, and seldom found at any very great distance 

 from land. They belong chiefly to the temperate 

 and the cold regions, in the latter of which they are 

 exceedingly numerous. They consist of only two 

 genera : seals, which are by far the most numerous, 

 both in species and in individuals ; and morses, which 

 are remarkable for the vast enlargement of the 

 canine teeth in the upper jaw. They also want both 

 the canines and the incisive teeth in the under jaw. 

 It does not appear in what particular way the morse 

 uses its immense canines, which are directed down- 

 wards from a very thick and square muzzle ; but it 

 has been supposed that it uses them partly in climb- 

 ing the rocks of those wild shores which it inhabits, 

 and partly in gathering sea-weed. 



In many of the popular books, and also in the older 

 systematic ones, this tribe of the carnivora has been 

 confounded with some of the cetacea. The difference, 

 however, is complete and obvious both in the exter- 



nal appearance of the animals and in their skeletons ; 

 and those cetacea which have been confounded with 

 the amphibia or vegetable feeders, are chiefly inha- 

 bitants of rivers. 



It was the old hypothesis that, in these amphibia, 

 the foramen ovate, or internal opening from the one 

 cavity of the heart to the other, remained open through 

 life ; and from this it was concluded, that if, in man, 

 or in any other land animal, this passage could be 

 kept open, they could rermiin as long under water as 

 a seal or a morse. The foundation is, however, phy- 

 siologically incorrect, and consequently the structure 

 cannot stand. The passage of the blood through 

 the heart, without going to the lungs, would not only 

 be of no use, but it would be fatal to any of the mam- 

 malia. After the blood has once circulated through 

 the system and is returned to the heart, it is quite unfit 

 for stimulating the systematic ventricle of that organ, 

 or of supplying the waste of life, until it has undergone 

 the action of the air and respiration. Those animals, 

 therefore, do not, any more than the land mammalia, 

 carry on their circulation through the heart while 

 they are under water, they can merely remain a con- 

 siderable time below ; and in this respect the otters 

 make an approximation to them, and they have also 

 valves for closing the nostrils, though less perfect 

 than the valves of the amphibia. 



We shall here venture to make a slight departure 

 from Cuvier's arrangement, and instead of considering 

 the marsupial animals as the fourth order in the 

 arrangement, we shall place them as the last. This 

 seems their proper station, if we are to make the 

 arrangement as useful as possible by making it struc- 

 tural, because these animals have a distinguishing 

 character, in an additional membrane and its support- 

 ing bones, which does not belong to any of the rest. 

 Farther than this, the marsupial animals do not form 

 one order according to any of the characters upon 

 which the other orders are established ; and therefore 

 it really would appear that these curious animals ought 

 to be made a supplement, or sub-class, to follow the 

 true mammalia. We merely mention this, however, 

 without recommending it to be adopted, for we are 

 not making a system, but performing the humbler 

 duty of attempting to explain one. 



RODENTIA. The animals of this order are, gene- 

 rally speaking, of small size, but they are exceedingly 

 numerous, found in one species or other in every part 

 of the world except New Holland, and sometimes in 

 numbers so great as at certain seasons to destroy the 

 whole vegetation. Some of the class are the most 

 gentle of the mammalia, and some are so ferocious 

 that, if their size and strength were in proportion, 

 they would be exceedingly formidable. Some of the 

 most curious in their habits also belong to this class, 

 of which we may mention the beaver that builds 

 houses, and there are also many which treasure up 

 stores of provisions. Generally speaking they are 

 more fertile than any other of the mammalia, as the 

 period of their gestation is shorter, and their litters 

 more numerous. 



But though the animals differ greatly from each 

 other in size, in external appearance, and in habits, 

 the order is still a very natural one, and true to the 

 character upon which it is founded, which character 

 is well expressed in the name. 



Rodentia,formed from the verb rorfo.to gnaw, means 

 gnawing animals ; and this is the action of the anterior 

 part of the mouth in the whale. The typical number 



