38 VERTEBRATE ANIMALS. 



respiration is moderate, are generuuy rormea to waik and run with precision and 

 vigour ; the birds, in which it is greater, have the muscular energy and hghtuesrs 

 necessary for flight ; the reptiles, where it is diminished, are condemned to creep, and 

 many of them pass a portion of their life in a state of torpor ; the fishes, in fine, 

 to execute their movements, require to be supported in a fluid specifically almost as 

 heavy as themselves.* 



All the circumstances of organization proper to each of these four classes, and 

 especially those which refer to motion and the external senses, have a necessary 

 relation with these essential characters. 



The class of mammahans, however, has peculiar characters in its viviparous mode of 

 generation, in the manner in which the foetus is nourished in the womb by means of 

 the placenta, and in the mammse by which they suckle their young. 



The other classes are, on the contrary, oviparous ; and if we place them together, in 

 opposition to the first, there will be perceived numerous resemblances which announce, 

 on their part, a special plan of organization, subordinate to the great general plan of 

 all the vertebrates. 



THE FIRST CLx\SS OF VERTEBRATE ANIMALS. 



MAMMALIA. 



Mammalians require to be placed at the head of the animal kingdom, not only 

 because this is the class to which we ourselves belong, but also because it is that which 

 enjoys the most numerous faculties, the most delicate sensations, the most varied 

 powers of motion, and in which all the different qualities seem together combined to 

 produce a more perfect degree of intelligence, — the one most fertile in resources, most 

 susceptible of perfection, and least the slave of instinct. 



As their quantity of respiration is moderate, they are in general designed for walking 

 on the ground, but with vigorous and continued steps. Consequently, all the articula- 

 tions of their skeleton have very precise forms, which rigorously determine their motions. 



Some of them, however, by means of lengthened limbs and extended membranes, 

 raise themselves in the air ; others have the limbs so shortened, that thej^ can employ 

 them with effect only in water ; but they do not the more on this account lose the 

 general characters of the class. 



^oups which they approximate in habit, — nought that can be regarded 

 as an intentional or designed rfpresentatitjit of them, as has some- 

 times been imagined : for it is evident, that if species based on two 

 ditTcrent plans of organization are respectively modi5ed to perform 

 the same office in the economy of nature, they must necessarily re- 

 semble, to a certain extent, superficially, as a consequence of that 

 adaptation ; while there are many cases also in each class which can- 

 not well be represented in some others, as that of the mole among 

 quadrupeds, which has no counterpart or correspondent group in the 

 ample of a genus of flying reptiles, the fossil remains of which only I class of birds. Habit, or mode of life, has indeed nothing whatever 

 have been discovered. Descending to lower groups, we find among to do with the physiological relations of organisms, which afTord the 

 birds, a genus of thrushes {Cindm) , which seeks its subsistence under 1 only legitimate basis of classification ; and those special modifications 

 water; and another of totipalmatc water-fowl (.Tnchypelrs) , which i to particular habits, which, occurring alike in any class, superinduce 

 neither swims nur dives. Such deviations, however, from the general ! a resemblance in superficial characters only, constitute what lias been 

 character of their allied genera, have no intrinsical relation to the j wcU distinguished by the term analogy, as opposed to aJ/iuitj/.—LD. 



