and substance. This, however, is owing, not to their continuing to possess life itself, but partly to the particular character of the texture of which they are composed, and partly to their protection, by accident or intention, from the influence of air, warmth, and moisture. Nothing like this takes place in other substances: they can be destroyed only by the action of some mechanical agent, which separates their parts, or by that of some chemical one, which alters their combinations.

There is still a fifth circumstance in which organized differ essentially from unorganized bodies. The properties which the latter possess, and the laws by which they are governed, are definite in their character. Uniform causes produce uniform results; and these results are capable of being calculated and measured. It is widely different in living bodies, so far at least as we are able to analyze the conditions under which they exist. This peculiarity is most distinctly exhibited among the higher classes of animals, but is to be everywhere recognized. Individuals of the same species differ indefinitely as to the mode and degree in which they are influenced by external causes. We can predict of any inorganic body, that it will be always affected in the same way by the same agents. We cannot predict this of any organic body; on the contrary it is hardly ever influenced in precisely the same way at different periods by the same cause.

A remarkable result of this peculiarity is the formation of habits. All living things are believed to be capable of this. No others are capable of it in the slightest degree. It is one of the most striking differences between the two great divisions of natural objects. It is exhibited in a variety of ways. The life of many, both plants and animals, is a history of the formation of habits, some of which are confined to, and terminate with, the individual, others are transmitted to the offspring. Thus the physical character and even structure of plants are altered by climate, by modes of cultivation, by kinds of food; and not only is the same true of animals, but it goes even further, and we see changes formed and transmitted, so to speak, in their instincts, their intellect, and even in their passions and propensities. Hence it is in the subjects of organic life alone that we discover the existence of a proper individuality.

There are other phenomena which further illustrate the distinction between the organic and inorganic modes of existence. Animals and vegetables are capable of passing into a condition in which there is a suspension, for a time, of many and sometimes of all their functions, which after a time they resume. Of this we have examples in the daily sleep of animals, and the torpid state into which many animals and almost all plants fall during certain portions of the year. There is, in these cases, an intermission in the exhibition of those properties by which the individual is particularly characterized. Nothing truly analogous to this takes place among mineral substances.

CHAPTER II.

CHARACTERISTICS OF ANIMALS AS DISTINGUISHED FROM PLANTS.

PRACTICALLY there is not often any difficulty in distinguishing an animal from a vegetable. But when it is attempted to point out the philosophical or essential principle in which their difference consists, the task is not so easy. In fact, there does not appear to be any such principle lying at the foundation of the distinction between animals and plants, that there is between organized and unorganized bodies, namely the principle of life. There are certain close points of resemblance between the composition, the structure, the functions, and the conditions of existence of animals and vegetables, which do not exist between either of them and minerals. So much is this the case, that some writers, among whom was the celebrated Buffon, have believed that there is no exact boundary, but that so close a resemblance of characteristics exists between those living at the two extremes, that individuals possessing the peculiarities of animal life in the lowest degree, are not essentially different from the plants which possess them in the highest.

It is probable, however, that the difference between these two classes of bodies is essential and fundamental; and it is worth while to enter somewhat into the examination of it, not simply on account of its intrinsic importance, but because such an examination will serve to illustrate not only the differences between animals and vegetables, but the nature of their life, the tenure and conditions of their existence, and the general character of the structure and functions by which their existence is originated and maintained.

It has been remarked that a vegetable may be compared to an animal asleep, since it exercises, throughout its whole existence, just those functions, and no others, which an animal continues to exercise during sleep. "Sleep," says Buffon, "which appears to be a state purely passive, a species of death, is, on the contrary, the original condition of animated beings, and the very foundation of life itself. It is not a privation of certain qualities and exertions, but a real and more general mode of existence than any other." This remark is more ingenious than just. It is founded on an imperfect view of the nature of sleep. The essential quality of this state is that it implies the suspension of certain functions during its continuance, which may be exercised at other times. A living body cannot justly be regarded in a state of sleep, which is in the actual performance of all the functions of which it is capable. Strictly considered, the remark means only this that, during sleep, animals continue to perform only those functions which are absolutely necessary to the existence of a living thing; that these functions are those which vegetables always perform; and that there is consequently an analogy between an animal, when its peculiar functions are suspended, and a vegetable in its ordinary state of existence. This analogy, though fanciful in the terms in which it is expressed, is founded upon the real differences between the two forms of life; and vegetable life, though not a more real, may justly be regarded as a more general mode of existence" than animal.

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The animal life seems to be, in fact, superinduced upon the vegetable. The fundamental operations of living systems - those by which they are brought into, and continued in existence are the same in both. They have been called, by way of distinction,

the vegetable or organic functions. Animals perform, in addition to these, certain others peculiar to themselves; chiefly reducible to the two, sensation and motion; and these are denominated the animal functions. It is in the structure by which, and the mode in which, these several functions are performed, that the real distinction between the two kingdoms is to be sought; and although there may be certain individuals in each, with regard to which it is difficult to detect the details of the distinction, there is reason to infer, from the well-established uniformity of the laws upon which the creation proceeds, that they exist.*

We find a marked difference between animals and vegetables, as to the manner in which the functions common to both are performed, as well as in the possession by the former of certain others, which are not possessed by the latter. To both, as already remarked, a male and female parent is necessary. In the vegetable, the new individual is produced by means of a seed; in the animal, of an egg; for though many animals are viviparous or produce their young alive, the process is in them essentially the same as in the oviparous, differing only in the circumstance that incubation takes place within the body of the mother.

The seed, then, corresponds to the egg, but there is a difference in the mode of their development. The organs of the animal are formed within the egg, and it is not extruded till it is capable of performing all those functions which are essential to its life. During this process it is nourished by the contents of the egg, or by materials derived from a subsequent continued connection

* A doubt has sometimes arisen in regard to certain species, whether they should be considered as belonging to the vegetable or animal kingdom. But the existence of this doubt does not involve any question as to the essential distinction between these kingdoms. In fact, the very controversy involves the recognition of their fundamental difference. It may be further observed that this doubt has always existed with regard to individuals at the very lowest boundary of the two kingdoms. Now it is in the highest forms at which the two ever arrive that we are to seek for the distinction between them, and here we readily detect sufficient evidence that this distinction is an essential one. If we cannot detect this among the lower and more obscure forms, we have reason to infer, not that it does not exist, but that from a deficiency in our means of observation it eludes the scrutiny.

with its parent. In the seed, on the contrary, although it contains the germ of the future plant, and the process of development begins within it, yet it mainly takes place without, and it is not till after it has sent its roots into the earth, and its stem into the air, that the organs necessary to its future existence are constructed.

Plants and animals are equally dependent upon food, for the continuance of existence and the performance of their functions; but they differ from each other in the manner in which it is done. In the plant it is effected by means of roots. These are usually distributed under the earth, but sometimes they float loosely in the water, are attached to other plants, or in some rare cases are only exposed to the air. In animals, on the contrary, food is received into an internal cavity, and undergoes the process of digestion, before it is admitted into the circulation, and applied to the nourishment of the system. The difference then is, that plants absorb their food by an external surface, whilst animals absorb it by an internal surface.

They differ also in the nature of their food. Animals derive their nourishment chiefly, if not exclusively, from matter which has been already organized, either in some vegetable or animal system. Plants, on the contrary, derive it chiefly, if not exclusively, from elementary matter, or matter which is not in an organized state. It is true that they flourish best in a soil which contains the remains of vegetables and animals; but it is only after these remains have lost their peculiar structure, and are in a state of decomposition, that they answer this purpose. A plant cannot subsist upon animal or vegetable substances as such, nor an animal upon simple unorganized matter.

They differ again in their chemical composition. To vegetable substances three elements are principally necessary, — hydrogen, oxygen, and carbon. Animals in addition to these require a fourth, azote or nitrogen. It is true that there are some important vegetable substances into whose composition nitrogen enters, and some animal ones into whose composition it does not; and that in addition to these principal ingredients there are certain secondary ones which are more or less constantly present in both, such as salt, lime, sulphur, phosphorus, iron, and some others.