46. The tissues above enumerated differ from each other more widely, in proportion as they are examined in animals of a higher rank. As we descend in the scale of being, the differences become gradually effaced. The soft body of a snail is much more uniform in its composition than the body of a bird or a quadruped. Indeed, multitudes of animals are known to be made up of nothing but cells in contact with each other. Such is the case with the polyps; yet they contract, secrete, absorb, and reproduce; and most of the Infusoria move freely, by means of little fringes on their surface, arising from a peculiar kind of cells.

47. A no less remarkable uniformity of structure is to be observed in the higher animals, in the earlier periods of their existence, before the body has arrived at its definite form. The head of the adult salmon, for instance, contains not only all the tissues we have mentioned, namely,

Fig. 8.

bone, cartilage, muscle, nerve, brain, and membranes, but also bloodves

sels, glands, pigments, &c. Let us, however, examine it during the embryonic state, while it is yet in the egg, and we find that the whole head is made up of cells which differ merely in their dimensions; those at the top of the head being very small, those surrounding the eye a little larger, and those beneath being still larger, (Fig. 8.) It is only at a later period, after still further development, that these cellules become transformed, some of them into bone, others into blood, others into flesh, &c.

:

48. Again the growth of the body, the introduction of various tissues, the change of form and structure, proceed in such a manner as to give rise to several cavities, variously combined among themselves, and each containing, at the end of these transformations, peculiar organs, or peculiar systems of organs.

SECTION III.

DIFFERENCES BETWEEN ANIMALS AND PLANTS.

49. At first glance, nothing would seem more widely different than animals and plants. What is there in common, for instance, between an oak or an elm, and the bird which seeks shelter amid their foliage?

50. The differences are usually so obvious, that this question would be superfluous if applied only to the higher forms of the two kingdoms. But this contrast diminishes, in proportion as their structure is simplified; and as we descend to the lower forms, the distinctions are so few and so feebly characterized, that it becomes at length dif ficult to pronounce whether the object we have before us is an animal or a plant. Thus, the sponges have so great a resemblance to some of the polypi, that they have generally been classed among animals, although in reality they be long to the vegetable kingdom.

51. Animals and plants differ in the relative predomi nance of the elements, oxygen, carbon, hydrogen and nitrogen, of which they are composed. In vegetables, only a small proportion of nitrogen is found; while it enters largely into the composition of the animal tissues.

52. Another peculiarity of the Animal Kingdom is, the presence of large, distinctly limited cavities, usually intended for the lodgment of certain organs; such is the skull and the chest in the higher animals, the cavity of the gills in fishes, and of the abdomen, or general cavity of the body which exists in all animals, without exception, for the purpose of digestion, or the reception of the digestive organs. 53. The well-defined and compact forms of the organs

lodget: in these cavities, is a peculiarity belonging to animals only. In plants, the organs designed for special purposes are never embodied into one mass, but are distributed over various parts of the individual. Thus, the leaves, which answer to the lungs, instead of being condensed into one organ, are scattered independently in countless numbers over the branches. Nor is there any organ corresponding to the brain, the heart, the liver, or the stomach.

54. Moreover, the presence of a proper digestive cavity involves marked differences between the two kingdoms, in respect to alimentation or the use of food. In plants, the fluids absorbed by the roots are carried, through the trunk and all the branches, to the whole plant, before they arrive at the leaves, where they are to be digested. In animals, on the contrary, the food is at once received into the digestive cavity, where it is elaborated; and it is only after it has been thus dissolved and prepared, that it is introduced into the other parts of the body. The food of animals consists of organized substances, while that of vegetables is derived from inorganic substances; and they produce albumen, sugar, starch, &c., while animals consume them.

55. Plants commence their development from a single point, the seed, and, in like manner, all animals are developed from the egg. But the animal germ is the result of successive transformations of the yolk, while nothing similar takes place in the plant. The subsequent development of individuals is for the most part different in the two kingdoms. No limit is usually placed to the increase of plants; trees put out new branches and new roots as long as they live. Animals, on the contrary, generally have a limited size and figure; and these once attained, the subsequent changes are accomplished without any increase of volume, or essential alteration of form; while the appearance of most vegetables Is repeatedly modified, in a notable manner, by the develop

ment of new branches. Some of the lowest animals, however, the polyps for instance, increase in a somewhat analogous manner, (§ 329, 330.)

56. In the effects they produce upon the air by respira tion, there is an important difference. Animals consume the oxygen, and give out carbonic acid gas, which is destructive to animal life; while plants, by respiration, which they in most instances perform by means of the leaves, reverse the process, and thus furnish oxygen, which is so essential to animals. If an animal be confined in a small portion of air, or water containing air, this soon becomes so vitiated by respiration, as to be unfit to sustain life; but if living plants are enclosed with the animal at the same time, the air is maintained pure, and no difficulty is experienced. The practical effect of this compensation, in the economy of Nature, is obviously most important; vegetation restoring to the atmosphere what is consumed by animal respiration, combustion, &c., and vice versa.

57. But there are two things which, more than all others, distinguish the animal from the plant, namely, the power of moving itself or its parts at will, and the power of perceiv ing other objects or their influences; in other words, voluntary motion and sensation.

58. All animals are susceptible of undergoing pleasure and pain. Plants have also a certain sensibility. They wither and fade under a burning sun, or when deprived of moisture; and they die when subjected to too great a degree of cold, or to the action of poisons But they have no consciousness of these influences, and surfer no pain; while animals under similar circumstances suffer. Hence they have been called animate beings, in opposition to plants which are inanimate beings.

CHAPTER THIRD.

FUNCTIONS AND ORGANS OF ANIMAL LIFE.

SECTION I.

OF THE NERVOUS SYSTEM AND GENERAL SENSATION.

59. LIFE, in animals, is manifested by two sorts of functions, viz.: First, the peculiar functions of animal life, or those of relation, which include the functions of sensation and voluntary motion; those which enable us to approach, and perceive our fellow beings and the objects about us, and to bring us into relation with them: Second, the functions of vegetative life, which are nutrition in its widest sense, and reproduction; those indeed which are essential to the maintenance and perpetuation of life.

60. The two distinguishing characteristics of animals, namely, sensation and motion, (57,) depend upon special systems of organs, which are wanting in plants, the nervous system and the muscular system under its influence. The nervous system, therefore, is the grand characteristic of the animal body. It is the centre from which all the commands of the will issue, and to which all sensations tend.

* This distinction is the more important, inasmuch as the organs of animal life, and those of vegetative life, spring from very distinct layers of the embryonic membrane. The first are developed from the uppe. layer, and the second from the lower layer of the germ of the animal See Chapter on Embryology, p. 112.