MAMMALIA. 



115 



of the two kinds of excitement, we invariably find that 

 sparks are most readily obtained from the cat's back in 

 those states of the atmosphere best adapted for the 

 working 1 of a common electric machine. The sensihi- 

 lity of many of the mammalia to atmospheric change?, 

 even when those changes are too slight for telling 

 upon our feelings or upon our instruments, are also well 

 known ; and in this way the almost eyeless mole, 

 lodged under the surface of the ground, is probably a 

 more certain " weather-wiser" than the most zealous 

 human student of the science of meteorology. Upon 

 this branch of the system, as well as upon the others, 

 we may therefore say that the system of sensation is 

 the same in kind among all the mammalia, and only 

 varies in degree in the different parts, so as to adapt 

 each species to its proper haunt and habit in the 

 most perfect manner. 



Even in the external coverings of their bodies there 

 is a characteristic resemblance among all the mam- 

 malia, in which they differ from all other animals, 

 and which, if it is known in the case of one can never 

 be mistaken in that of another. The skin of the 

 mammalia is very thin in some species, and very 

 thick in others ; but still in the whole it consists 

 of the same number of parts similar in their com- 

 position and their arrangement. Whatever appen- 

 dages grow from this skin for protection to the 

 animal, or for any other purpose, are always substan- 

 tially the same. They may be of exceedingly delicate 

 texture as we find in many of the fur animals, and 

 in the fleeces of some of the finer-woolled sheep, or 

 they may be coarse and shaggy, of which we have 

 examples in the produced hair of the American bison 

 and the mane of the lion. They may also have that 

 appearance which we find in the substance called 

 whalebone, or more correctly baleen, which forms the 

 apparatus used by toothless whales for capturing their 

 food ; it may be tubular, with an opening in the centre, 

 as we find it in the ornithorhynchus and some other 

 animals ; it may be in hard spines, which have their 

 insertion into cavities, as in the porcupine, the echidna, 

 the spiny rats, and some others ; it may be an 

 agglutination of fibres without any core, as we find in 

 the horn of the rhinoceros, or it may be a casement 

 of horny matter upon a core of bone, as we find in the 

 ox, and all those other animals which have true horns ; 

 and, according to the species, it may differ in its con- 

 sistency, and also in the proportions of its ingredients, 

 but it is still substantially the same substance ; and 

 whether it appears in the form of fur, of wool, of 

 hair, of bristle, of horn, of hoof, or claw, or of nail, it 

 is still substantially the same, and when examining it 

 we could at once pronounce it to be a product of the 

 skin of the mammalia in one or.other of the species, 

 and not any thing else. 



Thus, notwithstanding all the diversity which there 

 is in the size, the structure, and the habits of this 

 class of animals, there is in every part of them a 

 general character which prevents us from confounding 

 them with any other animals ; and this character 

 pervades their whole organisation, and is traceable in 

 all their habits. 



In the mode of their reproduction the means 

 which have been given them for the continuation of 

 their several species, there is the same uniformity. 

 Every species consists of two sexes, and each indivi- 

 dual is of one sex only. Instances are indeed men- 

 tioned of the appearance of a double sex ; but these 

 instances, which are rare, are always monstrous form- 



ations, and whatever may be their appearance they 

 never have a double function, and very often they 

 have no function at all. Some physiological facts 

 connected with this part of the subject are very curi- 

 ous, but at the same time perfectly inexplicable upon 

 any principles with which we are acquainted. One 

 of the most familiar, and at the same time perhaps 

 one of the most singular, is the case of" free martens." 

 Hitherto we believe the occurrence of them has been 

 found in no animal save the common domestic ox, 

 and no reason can be assigned for its occurrence in 

 that animal. One young one is understood as being 

 the regular or normal number produced at a birth in 

 this species of animal, but there are sometimes two. 

 If these two are both males or both females, they follow 

 the ordinary law, and are perfect animals ; but if the 

 one of them is a male and the other a female, the female 

 is imperfect or a free marten ; and differs riot only in 

 its general expression but in its flesh from the peri'ect 

 animal, whether male or female, being accounted far 

 superior to either of them as food, even though they 

 are mutilated very soon after their birth. 



There is another remarkable distinction in the mode 

 of reproduction in the mammalia, and that is the dif- 

 ference between placental animals and marsupial 

 ones. The latter have not yet been completely inves- 

 tigated ; and for much of what is known respecting 

 them we may refer the reader to the article KANGA- 

 ROO. But, independently of the difference of structure 

 requisite for the double gestation of marsupial animals, 

 there is a characteristic distinction of a more general 

 kind between them and the placental mammalia, that 

 bring forth their young in full maturity. Their appear- 

 ance as to form wants the elegance and lithencss of 

 the others. They are shapeless animals so so speak, 

 and the very covering of their bodies is different, 

 though it is not very easy to speak particularly of 

 what the difference consists. The most remarkable 

 distinction between them and the others, however, 

 is in the expression. The eyes of all marsupial ani- 

 mals look blank and vacant, and though physiognomy 

 is not perfectly understood, and consequently our 

 opinions concerning it cannot always be trusted, yet 

 the whole air of the head, as well as the general 

 shape of the body, give us at once the impression 

 that marsupial animals are stupid and resourceless. 

 Even the feet, which one would suppose to be less 

 affected by physiological differences than any other 

 parts, are different from those of the common or cha- 

 racteristic mammalia, and in many of the species they 

 bear some resemblance to the feet of birds or those 

 of reptiles. The anatomical structure has not been 

 examined with the requisite degree of attention ; but 

 it should seem that there is an inferiority in the 

 tarsal and carpal parts of the extremities, and that the 

 foot, for whatever kind of work it may be adapted, is 

 always inferior to that of the placental mammalia. 



But, notwithstanding the differences of appearance, 

 expression, and intelligence, which follow this dif- 

 ference in the production of these animals, their 

 predominating character is still that of the mammalia, 

 and therefore in what light soever we view the numer- 

 ous species which constitute the class, we must always 

 come to the conclusion that it is clear and well defined 

 in its general characters. 



SEC. II. PHYSIOLOGY or MAMMALIA. Physiology 



literally means "the voice of nature," but in general 



it is restricted to nature as organised, and thus it 



becomes " the voice of life," and is divided into two 



H2 



