62 INTRODUCTION. 



guishes itself from instinct, a certain capability of instruction, 

 &c. ; it is not only the influence these organs hold over the 

 others, in order to direct their action, but it is, above all, the 

 concentration of life in the central or predominant organs 

 in the heart, in the nervous centre, and in the action of these 

 parts on each other. Even in this point of view, however, 

 there is a great difference among the vertebrata. 



56. The vertebrate animals which are so much alike in 

 most of their characters, present, in fact, a very considerable 

 difference. The similitude is particularly strong in the cen- 

 tral part of the nervous system, and in its envelope, that is to 

 say, in the spinal marrow, and spine; and their differences in the 

 extremities and at the surface, as in the brain, the cranium, the 

 organs of sense, the face, the organs of motion, themembers, 

 and skin. In the same way, among the organs of the vegetative 

 functions, the heart presents many differences, but they are 

 particularly great in the organs and phenomena of respira- 

 tion; and as the action of the muscles and of the nervous sys- 

 tem depends greatly on respiration, variations in this function 

 occasion corresponding ones in the animal function. Thus in 

 the mammalia, where the circulation is double, that is, where 

 all the blood brought from the body is sent to the lungs be- 

 fore returning to it, and where respiration is aerial, the mus- 

 cular action is strong. In birds, where the circulation is dou- 

 ble, and where respiration is aerial also, is not confined to the 

 lungs, but extends itself to various parts of the body, the vi- 

 gour of the muscles is still greater; in reptiles, where the cir- 

 culation is simple, and respiration consequently partial, it is 

 weak and the movements slow, since a part only of the blood 

 is submitted to the action of the air, previous to its return to 

 the body. Fishes, it is true, have a double circulation, but 

 their respiration can not be complete, on account of the small 

 quantity of air contained in the water they respire, conse- 

 quently, as to station, they are nearly in a state of equilibrium 

 in water. Animals of the first two classes have much warmer 

 blood than those of the two latter, which on this account are 

 called the cold blooded vertebrata. 



Their mode of generation, presents also a remarkable differ- 



