scribed with regard to the right side. The left ventricle, from its superior size and strength, gives to the blood a more powerful impulse than that which it receives from the right. This is neces-¡ sary, because it has a wider and more extensive course to traverse. From the left ventricle, it is thrown into the aorta, the great artery which supplies the whole body with blood. This artery ascends from the heart for a short distance, arches over, sends branches to the head and arms, and then descends behind the heart, and distributes branches to the other parts of the system.

The branches thus distributed throughout the body are subdivided again and again to an almost inconceivable degree of minuteness, and finally terminate in a system of vessels called capillary vessels. These pervade every part; and the blood, after passing through them, enters into another set of vessels, the veins, which gradually collect together and enlarge in size, till they terminate, as has been before remarked, in two large trunks at the right auricle of the heart. In the capillary vessels, the blood undergoes a change in its qualities, precisely opposite to that which takes place in the lungs. It becomes, from a bright red color, of the same dark red which it was described to possess when passing through the right side of the heart. This change is presumed to proceed from the office which the blood performs in the nutrition of the body during its circulation, and by which some of its elements are abstracted from it, and combined with the texture of the organs.

This office, namely, the nutrition of the several parts, consists of a double operation. By the first, the particles of an organ which have become unfit any longer to enter into its composition, are taken up and carried back into the circulation. This process is called ABSORPTION; by the second, called NUTRITION, new particles are substituted in their place. In this way the injury sustained by the wear and tear of the system is constantly repaired, and each part is kept fit for use.

The particles which thus go back into the blood, and circulate with it, are not, however, retained in the system for any length of time. They are speedily separated by a process called EXCRETION, and are carried off chiefly by the skin, the lungs, and the kidneys.

CHAPTER IV.

STRUCTURE OF ANIMALS AS COMPARED WITH THAT OF MAN.

ALTHOUGH it is no part of the plan of this work to enter into a detailed account of the Natural History of animals, especially of their scientific arrangement, still it will be necessary to give some general description of the system of classification usually received among naturalists, and of the principles on which it is founded.

The functions briefly described in the last chapter, by which life is maintained, are few in number and definite in purpose. They are essentially present in all animals: that is to say, it is necessary to all that their systems should be supported by blood, which is effected by a CIRCULATION of some kind, however simple; that this blood should be regularly exposed to the influence of the air by RESPIRATION; that the effete particles should be removed from the textures by ABSORPTION, and from the body by SECRETION and EXCRETION, and that they should be restored to the blood by DIGESTION. It is necessary, also, that they should have a perception of objects external to themselves by means of SENSATION, and the power of moving the parts of their body by the will or LOCOMOTION. These functions are present in all, though performed in a much more limited manner in some than in others; so that,—notwithstanding this essential identity of function, in no two is there the same form and structure. They exhibit an almost endless variety in the manner in which the objects of life are accomplished, and out of this grows their almost endless variety in external form, and in some degree, though to a far less extent, in their internal organization.

This variety seems intended to adapt them to different residences, to live in different mediums, and for their universal distribution over the surface of the earth. It is thus that quadrupeds are fitted for the land, birds for the air, and fishes for the water; and it may be taken as a fundamental principle, that the varieties

of external form and internal organization which animals exhibit are intended to adapt them for something in those particular external circumstances in which they are placed. The reason is obvious enough why this should be necessary with regard to their organs of sense and motion, but not so clear with regard to their internal structure. Yet it is not less so. A correspondence is found to exist between all the parts of an animal. It may be primarily necessary, in order to adapt an individual species to its destined residence and kind of food, that only a single set of its organs should be modified, but then, on account of the intimate relations and reciprocal dependence of the functions, it becomes secondarily as necessary that some modification take place in every other organ, in order to that perfect harmony of action which Nature requires.

Suppose we substitute for one of the wheels of a watch, another which has a single tooth less. By even so slight an alteration as this, the whole movement of the machine will be deranged. It will cease to keep exact time, because the correspondence between all its parts has been destroyed. This can only be remedied by remodelling every other part so as to restore this correspondence. Here, perhaps, we must substitute a smaller wheel, and there a larger; this lever must be longer, and that shorter; and so on till the movements are again equalized, and harmony restored. Similar modifications are required in the more delicate and more nicely adapted machinery of which the bodies of animals are examples. Alter the mode of performing one function, and you disturb its harmony with the others, unless at the same time you introduce a corresponding modification into them.

This would be in fact to make a new animal. It is in this way, indeed, that Nature makes new animals, or, to speak more exactly, in this way she has been able, out of a limited number of organs and functions, to produce the immense variety of kinds with which her dominions are filled.

When an animal is to be constructed to live upon food not very unlike that of man, but to seek it in a different way; in a particular part of the earth, where it is presented to him spontaneously through the whole year, and therefore requiring a less varied intelligence; obliged to ascend trees, partly to procure

this food and partly to escape his natural enemies,

- Nature does not invent a new plan, but makes certain necessary alterations in her old one. She takes man. She shortens and weakens his legs, but lengthens and strengthens his arms. She takes away his feet and gives him hands upon all four of his extremities, less perfect than those of his prototype, but as perfect as his purposes require. She sharpens some of his senses, but diminishes his intellect. She gives him, therefore, a larger face with a smaller head, and then, after a variety of changes in correspondence with these, she has converted a man into a monkey.

Something analogous to this we observe among mankind in a variety of their works of art. From time immemorial, man has navigated the ocean in vessels impelled by the wind, and he has constructed them accordingly. But a new agent of motion, steam, is discovered, and this he desires to apply to the propelling of vessels on the water, as well as of machinery on land. He does not cast aside his old model, but modifies it and adapts it. With the same general shape and framework, certain parts are made stronger to endure the weight and motions of the ponderous machinery; other parts weaker because they have no longer to endure the strain of the masts and sails; some parts formerly devoted to other purposes must be now devoted to the stowing of fuel. Then the construction varies according as the vessel is intended to navigate a river, a lake, or the ocean, to be used in commerce or in war. In short, a new kind of structure is developed, in which all the parts are modified according to the new purpose. With our limited faculties this is only accomplished after long experience, many mistakes, many disasters, and, after all, the work is clumsy, and the harmony of parts imperfect, when compared with those examples of the same process with which the Creator has so profusely surrounded us.

It is by the pursuance of this plan that all Nature has been made full of life. Were there only a single form of animal existence, the earth would be for the most part untenanted. Man occupies but a small part of its surface and consumes but a small part of its productions. By the multiplication of the forms of life, there is an animal for every place and a place for every animal. As a general rule, there is no waste of material; the

variety of life furnishes a consumer for every product. The deer feeds upon grass, and is seized by the wolf; the wolf dies, and furnishes food for a thousand insects; these also are devoured in their turn, whilst all those vegetable and animal products not thus directly appropriated to the support of life, in their decay indirectly subserve to a more luxuriant vegetation, and reappear in new forms to answer the same purposes.

Thus it is that a great variety of kinds among animals is a necessary provision in a world constituted as this is; the final cause being that no room should be unoccupied, but that life and enjoyment should pervade every part. It follows that there will always be found a correspondence between the manner in which the ends of life are accomplished in each species, and the sphere in which that species is intended to move. Naturalists, in their examination of the animal kingdom with reference to its history and methodical arrangement, have always reference to this correspondence, and it is expedient, in order to the clearest illustration of the subject, to state somewhat more minutely the principles on which this correspondence of structure to purpose is regulated. In order to this, we may take one of the functions most immediate to life and see how its organ, and the mode and the quantity of its performance, are modified in this way.

One of the most important circumstances of external condition is the medium in which an animal resides, air or water. It renders necessary some modifications in shape, organs of motion, senses, covering, weapons of defence, &c., but more especially it renders necessary an appropriate structure of the organs of respiration.

Animals which breathe air have lungs, into the interior of which air enters, and, through their internal surface, exercises its influence on the blood. Animals that breathe in water have gills, or branchiæ, over the outside of which the water is made to pass, and, by means of the air that it contains, influences the blood through their external surface.

As the quantity of air contained in water is small, the quantity of blood renovated by its influence in a given time will be small, unless the organs of respiration are made very extensive. But his is not necessary to the condition of aquatic animals, and cou