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OUTLINES

OF

NATURAL HISTORY.

CHAPTER I.

INTRODUCTORY.

BEFORE Commencing the proper subject of this work-namely, a consideration of the leading types of structure as exhibited in the different classes of animals--it may be well briefly to consider how an animal lives. Without entering into any consideration of the much-vexed question whether the substance which composes the bodies of animals is in all cases substantially the same or not, it remains certain that the mere act of living is, in all cases, attended with more or less destruction of the material of which the living body is composed. Every vital act, of whatever kind, is attended by a destruction of the tissue which is concerned in the act. Thus, every movement of the body is effected at the expense of some muscle, and every thought we think is attended by a destruction of a portion of the substance of the brain. Hence comes the notorious fact that no animal can exist without food.

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If vital actions did not take place at the expense of the already existing substance of the body, then an animal, when once produced, might go on living for an indefinite period, without any necessity of eating. As it is, the process of living is attended with a constant destruction of the living matter of the body, and if this destruction were not counterbalanced in some way, an animal would rapidly waste away and die from the incessant losses of matter, caused by its movements and other vital processes. This result, however, is prevented by the fact that every animal is constantly taking in "food," for the purpose of repairing the losses caused by living, and in this the process of nutrition" (Latin, nutrio, I nourish), essentially consists.

Different animals live upon different kinds of food, but in all cases the food must contain materials which are capable of taking their place in the structure of the animal; otherwise it is not food in the proper sense of the term. And, as a matter of fact, the food of all animals, whether it consist of vegetables or of flesh, or of both combined, can be shown to consist of essentially the same constituents, and to be capable of entering into the body of the animal to be nourished. The first condition of nutrition, therefore, is that the animal should be able to get food containing materials which can be built up into its own tissues.

As a general rule, the food which an animal eats cannot be built up directly and without change into the new structures of the body. On the contrary, the food has to undergo certain changes before it can be employed in making new tissues. These changes are effected by what is commonly called the "digestion" of the food, but what is more properly and in a more general sense known as "assimilation" (Latin, assimilo, I make like to). In other words, the food has to be reduced to a common basis having a certain likeness to the tissues which it is intended to replace, before the animal is in a position to make use of it.

In most animals, the process of assimilation is com

menced, and to a certain extent carried out, by a more or less complicated digestive apparatus. In any case, the first part of assimilation consists in the melting down of the food into a common nutritive fluid, which contains certain organic compounds which existed in the food to begin with, or were manufactured out of it in the process of digestion. In the higher animals, the food is so acted upon in the stomach and intestines that it forms a complex fluid, which is then "absorbed" or sucked up from the alimentary canal, to form the "blood." The blood, therefore, is to be regarded as an organic fluid which is manufactured out of the food which the animal eats. The blood has dissolved in it the materials which are necessary for making new tissues, and it has to be distributed to the various parts of the body, so that each tissue may take from it the substances which are requisite to repair its losses. This is usually effected by a distinct propulsive organthe "heart"-which drives the blood to all parts of the body.

What occurs, then, in any of the higher animals, is readily understood. The various parts or tissues of the body are gradually wearing away, and they require fresh material for replacing their waste. This fresh material is contained in the blood, and is derived from the food; and it is incessantly driven to the different parts of the body. As the blood, therefore, circulates through each organ of the body, that organ abstracts from its living current the materials required to restore its losses and its waste.

Not only does each organ of the body take from the blood the materials requisite to repair its losses, but each at the same time throws off into the blood the worn-out and useless materials which have been produced by its slow destruction and wearing away. The result of this is that the blood very rapidly becomes impure, and gets loaded with waste matter as it circulates through the tissues and organs of the body.

It follows from this that the blood has two processes to undergo, if it is to remain in a healthy condition. In the first place, the various tissues of the body are con

stantly draining away from it nutritive materials, and it has therefore to be constantly receiving a supply of these same materials from the food, as prepared by the digestive organs. In the second place, the various tissues of the body are constantly throwing off into the blood waste matters, and the blood must get rid of these, if it is to retain its purity. This latter process is generally effected by means of distinct breathing organs, in which the blood is exposed to the action of the air. In the air-breathing animals the breathing organs are filled with air directly; in the water-breathing animals the breathing organs are adapted for absorbing the air which is generally dissolved in water. In either case, the waste materials contained in the blood are got rid of by the action of the oxygen contained in the air, which unites with them, and literally burns them up.

The main function, then, which any animal has to perform, is to nourish itself, or, in other words, to supply itself with materials capable of taking the place of the tissues which become worn out in the discharge of the vital functions. Animals, however, have more to do than this. They have to become acquainted with what is outside themselves, and to hold certain relations with the external world; and these relations come in a general way under the heads of "locomotion" and "sensation." Most animals, namely, can change their relations to external objects, as regards space, by moving, and their movements are generally effected by means of special locomotive organs. Most animals possess the power of changing their place bodily, and even those which cannot do this, by reason of their being permanently rooted to one place, can nevertheless move their bodies more or less freely. The organs of locomotion, or rather the agents of locomotion, are usually "muscles;" but some animals move about without having any distinct muscles, or indeed any permanent locomotive organs. In most cases, also, the organs of locomotion are brought under the control of the animal's will by means of the special structures which constitute a nervous system." It is not, however, by

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any means necessary that a distinct nervous system should be present.

As a general rule, animals can not only change their place, but they are also enabled to obtain some knowledge of the nature and characters of bodies outside themselves, by means of certain "organs of the senses." Thus, the higher animals have special organs for the perception of light, sound, odours, and tastes or flavours, whilst their sense of touch enables them to detect the more material properties of bodies foreign to themselves, as well as of their own bodies. As a general rule, these senses, as in the case of the locomotive organs, are under the control of a "nervous system;" but there are many animals in which a nervous system is absent, and to which we have, nevertheless, no reason to deny the possession of at any rate some of the senses, if in only a rudimentary form.

For the preservation and continued existence of each individual animal it is only necessary that it should be able to nourish itself, and that it should have certain relations with the outer world. Under any circumstances, however, the life of each individual animal comes to an end some time or other, owing to the inevitable failure of the nutritive powers which comes on when the animal has reached a certain age. In other words, death comes sooner or later to each individual animal, in consequence of its reaching a period of its life when it is unable to "assimilate" food enough to replace the constant loss of tissue caused by living. In every kind of animal, therefore, the individual dies after a longer or shorter period of existence, but the kind or "species" of animal does not die or disappear, because animals are endowed with the power of reproducing their like. Thus, by producing young like themselves, all animals have the power of continuing their kind through successive generations, in spite of the fact that the individuals of each generation perish, each in his appointed time.

It follows from what has been here remarked, that an animal, in its life, discharges three principal sets of functions. It nourishes itself; it has certain relations with

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