38 VERTEBRATE ANIMALS. 



respiration is moderate, are generally rormea to wane and run with precision and 

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

 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 mammalians, 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 mamma? 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 CLASS 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 they can employ 

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

 general characters of the class. 



* To descend to particular eases, however, it would appear that 

 species may be framed on almost every type, even very subordinate 

 types, for any particular mode of life. Thus, to illustrate briefly, the 

 bats, which are true mammalians, are modified for aerial progression 

 like birds ; and the whales, other mammalians, have a fish-like exterior, 

 being designed to live exclusively in water : so there are birds which 

 are utterly incapable of flight ; some, as the ostrich, adapted to scour 

 the plains, like a quadruped ; others, as the penguins, whose only 



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

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

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

 different plans of organization are respectively modified 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 



sphere of activity is in the water: the pterodactyle affords an ex- i quadrupeds, which has no counterpart or correspondent group in the 



ample of a genus of flying reptiles, the fossil remains of which only 

 have been discovered. Descending to lower groups, we find among 

 birds, a genus of thrushes {Cinvlus), which seeks its subsistence under 



class of birds. Habit, or mode of life, has indeed nothing whatever 

 to do with the physiological relations of organisms, which afford the 

 only legitimate basis of classification ; and those special modifications 



■water; and another of totipalmate water-fowl (Tachypetes), which j to particular habits, which, occurring alike in any class, superinduce 

 neither swims nor dives. Such deviations, however, from the general ! a resemblance in superficial characters only, constitute what has been 

 character of their allied genera, have no intrinsical relation to the ' well distinguished by the term analogy, as opposed to affinity. — Kp. 



