VERTEBRATE ANIMALS. 
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respiration is moderate, are generally formed to walk and run with precision and ^ j 
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, i 
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 , j 
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 t 
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. i! 
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 cases, 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, whicli 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 
sphere of activity is in the water : the pterodactyle affords an ex- 
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 (Ciwclus), which seeks its subsistence under 
water; and another of totipalmate w'ater-fowl {Tachypetes), which ] 
neither swims nor dives. Such deviations, however, from the general j 
character of their allied genera, have no intrinsical relation to the | 
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 
quadrupeds, which has no counterpart or correspondent group in the 
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 
to particular habits, which, occurring alike in any class, superinduce 
a resemblance in superficial characters only, constitute what has been 
well distinguished by the terra analogy, as opposed to affinity 
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