THE PECULIAR FACULTIES 277 



Sexual reproduction is thus peculiar to certain animals and plants : 

 consequently the simplest and most imperfect living bodies cannot 

 possess any such faculty. 



Circulation. This is a faculty which only exists in certain animals, 

 and which is much less general in the animal kingdom than the five 

 others of which I have already spoken. This faculty springs from an 

 organic function whose purpose is the acceleration of the movements of 

 the essential fluid of certain animals, — a function which is performed 

 by a special system of organs adapted to it. 



This system of organs is essentially composed of two kinds of 

 vessels, viz. arteries and veins, and almost always in addition a 

 thick and hollow muscle, which occupies about the centre of the 

 system, which soon becomes the principal motive power of it, and 

 which is called the heart. 



The function carried out by this system of organs consists in driving 

 the animal's essential fluid, which is here known by the name of blood, 

 from an almost central point occupied by the heart (when there is 

 one), through the arteries into every part of the body ; whence it returns 

 to the same point by the veins, and is then dispatched anew throughout 

 the body. 



It is this movement of the blood, always being driven into every 

 part and always returning to its starting point throughout the duration 

 of life, that has received the name of Circulation. It should be qualified 

 as general in order to distinguish it from the respiratory circulation, 

 which is undertaken by a special system likewise composed of arteries 

 and veins. 



Nature, when initiating organisation in the simplest and most im- 

 perfect animals, was only able to give their essential fluid an extremely 

 slow movement. This no doubt is the case in the very simple and 

 scarcely animalised essential fluid that moves in the cellular tissue of 

 infusorians. But afterwards she gradually animalised and developed 

 the essential fluid of animals in proportion as their organisation be- 

 came more complex and perfect ; and she accelerated its movement 

 by various methods. 



In the polyps, the essential fluid is nearly as simple and has scarcely 

 more movement than that of the infusorians. The regular shape of 

 the polyps, however, and especially their alimentary cavity begin to 

 furnish means to nature for somewhat increasing the activity of their 

 essential fluid. 



She probably took advantage of this in the radiarians, to estabhsh 

 in their alimentary cavity the centre of activity of their essential 

 fluid. The expansive surrounding subtle fluids, in fact, which consti- 

 tute the exciting cause of these animals' movements, penetrate chiefly 



