It is difficult to define the word animal, and even a scientific definition distinguishing an animal from a vegetable is scarcely less so. The assertion of Linnaeus, that " plants live and grow," while "animals live, grow, and feel," is probably correct; but it is impossible to verify its correctness as applied to the very lowest animals. According to the German naturalists, an animal may consist of a single cell. The idea also till recently maintained, that all animals have a stomach, or internal digestive cavity, is untenable; since many microscopic animals have no trace of a digestive apparatus. Indeed, there is no part or organ common to all animals. The stomach, the heart, and other parts of the circulatory apparatus, the mouth, and even the head, so indispensable in the higher animals, not only in the lower become modified in form and development, but in the lowest even entirely disappear. Nor can muscular fibres or nervous filaments be identified in the latter. - The changes in form of the same organ in different animals can merely be alluded to here. The mouth, for instance, usually single, and opening transversely, is sometimes double, triple, or multiple, and modified into a trunk, or sucker, as in many insects.

The heart has but two cavities in fishes, while there are three in reptiles, and four in birds and the mammalia. The same type of structure, however, often extends over a vast number of species of animals. All the vertebrate animals have the same typical skeleton, it being modified in the various species to suit their requirements. The bones of the anterior extremities, for instance, are the basis of the arm and hand of man, of the fore legs of quadrupeds and reptiles, the wings of birds, and the anterior tins of the whale. - Nor are our investigations in regard to function in all cases more satisfactory. Feeling and voluntary motion are certainly characteristic attributes of all but the lowest animals; but some of the latter are endowed with only a kind of motion of a lower grade than the voluntary, and do not give certain evidence of feeling at all. It is therefore not surprising that it was found impossible for a long time to distinguish the lowest animals from the simplest vegetable organisms; and that to these doubtful structures the name of zoophyte, or animal-plant, was given. Hence it is not proposed here to attempt to give a precise definition of the term animal, but only some of the more striking particulars which distinguish animals from plants.

It should, however, be here remarked, in regard to the microscopic animals, or animalcules, that Prof. Agassiz has shown that many of them are merely the ova, or germs of higher animal forms. - 1. An organism manifesting the power of sensation or voluntary motion, or possessing a digestive cavity (stomach), or into whose structure enters the nervous or the muscular tissue, is an animal. But, on the other hand, the impossibility of demonstrating either or all of these characteristics does not prove the organism to be a plant, as before stated. Besides, some plants, as the sensitive plant {mimosa pudica), withdraw their leaves from the touch of the hand; but not, as there is every reason to believe, in consequence of either sensation or volition. 2. Albumen is the great nutritive element of animals, while starch is that of plants. Some of the lowest plants (fungi) are, however, apparent exceptions to this proposition. Consequently, the chemical composition of the tissues of animals differs from that of plants: the basis of vegetable structure is cellulose, a compound of carbon, hydrogen, and oxygen, while those of animals contain nitrogen also in addition.

Recent researehes show that cellulose is found in some of the lower protozoa. 3. It has been stated that plants absorb carbonic acid gas from the atmosphere, and give out to it pure oxygen, while animals precisely reverse the process. Plants actually, in their nutrition and growth, assimilate the carbon of the carbonic acid of the atmosphere, and return its oxygen to the latter; but in the respiratory process they, like animals, consume the oxygen of the air, and return to it carbonic acid gas. By day, however, they give off less of the latter than of oxygen. 4. For the fixation of carbon in the tissues of plants, as just stated, the constant stimulation of light is indispensable. This is not true to the same degree of animals, whose tissues also consist in part of carbon, as has been seen. - In respect to varieties in size, the animal kingdom presents a far wider range than the vegetable. The extremes in the former are the whale, sometimes 100 feet long and weighing as many tons, and the animalcule, of some species of which 30,000 individuals may inhabit a single drop of water; while in the latter we find on the one hand the sequoia of California, 90 feet in circumference, and the talipot of Ceylon, a single leaf of which may shelter 20 men from the rain, and on the other the microscopic fungi, as the yeast plant (torula cerevisiae), or those constituting the mould on decaying substances.

Dick calculates that the largest trees of Guiana are 2,985,984,000,000,000 times as large as the rose-leaf plant; while the largest whale is to the minutest animalcule as 34,500,-000,000,000,000,000 to 1. - The number of species, and probably of individuals, is also far greater in the animal than in the vegetable kingdom. About 70,000 species of plants may be seen in Paris in a single collection. Balbi 40 years ago estimated the whole number of known species at 80,000; and it has been supposed that there are about 250,000 species in all on the globe. On the other hand, there are at least 100,000 species of animalcules alone. Dick estimated the whole number of species of animals at 300,000, and the number of individuals at 24 billions; while the parts and adaptations of these exceed 60,000 billions. - In regard to rapidity of increase, the highest plants vastly excel the highest animals. An elm of average size sometimes produces 158 million seeds. But the lowest animals and plants manifest the greatest power of multiplication.