26 



VEKTEBRATE ANIMAXS. 



Div. 1. 



respiration is moderate, are generally lormea to walk and run with precision and 

 vi'^our ; the birds, in which it is greater, have the muscular energy and lightness 

 necessary for flight ; the reptiles, where it is dimipished, 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, liowever, 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 jDlacenta, and in the mamrape 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 Avhich 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 diff'erent 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 witli effect only in water ; but they do not the more on this account lose the 

 general characters of the class. 



• To clcircnd lo pnrliculiir cusps, however, it woulil nppcar tliat 

 •pcclci may be franieil on almost every type, even very subonlinute 

 types, f.ir any particular mode of life. Tlius, to illustrate briefly, the 

 hall, whleli are true mammalians, arc mndifieil tor ai rial progression 

 liitc birtls ; anil tlie whales, otbcr mammalians, have a fish-like exterior, 

 telnit ile«l){neil lo live exclusively in water : no there are birils which 

 arc utterly incapable of flight ; tome, as the oslrith, ailapteU to scour 

 the plains, lilic a quadruped ; others, as the pen^'uins, whose only 

 •phrrc of activity is in the water: the pterodaelyie alTorils an ex- 

 ample iif a ircnus of flyini reptiles, the fossil remains of which only 

 have been discovered. DctccndinK to lower groupi, wc find among 

 birds, a genus of thrushes (Cincfiij) , which set ks its subsistence under 

 water; ami another of tutipalmate water-fowl (^Tnchyprtrt), which 

 neither swims niir dives. Such deviations, however, from the general 

 chancier of their allied genera, have no intrtnilcal relation to the 



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

 as an intentional or designed repri'setitntiun of tliem, as has some- 

 times been im.'igined : for it is evident, that if species based on two 

 dilTcrent plans of organization are respectively modified to perform 

 the same oflice 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 otliers, 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 witli tiie physiological relations of organisms, which afford the 

 only legitimate basis of classirication ; and those special modifieationg 

 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 lo affinity.— V.o, 



