Animal
ordinarily an organism that grows, lives, and feels. It is difficult to give a definition that will separate all animals from all plants. It is easy to point out the difference between the higher animals and higher plants. The horse and the cow, for instance, are different from the elm and the rose; but it is difficult to distinguish between the lower animals and the lower plants. Scientists have long been in doubt whether to call bacteria plants or animals. It is only of late that the decision has been given that they are plants. Both animals and plants grow, so growth does not help us out. It will hardly do to say that an animal feels and that a plant does not, for certain plants seem to be as sensitive as some animals. The sundew and other insect-catching plants are extremely sensitive. Most animals move about, yet a sponge remains fixed. Some large aquatic plants float about without sending their roots into the soil. Many microscopic plants seem to have power of moving toward food.
At their beginning, then, the animal and vegetable kingdoms seem quite on a level, and are almost indistinguishable; but the animal kingdom, with man at its head, far out-tops the vegetable kingdom. By scientists animals are distinguished from plants principally by the difference in the nature of the food of each. Animals can live on substances not differing much, if at all, from the substances composing their own bodies. They can also use solid food. Plants take their food in the form of gases or fluids only. A certain number of low forms seem to combine the characters of animals and plants, and the classification of these forms can be determined only after a special study of each case.
However much animals may differ among themselves, they are all alike in certain fundamental respects. All animals are composed of a complex living proteid substance, called protoplasm. This living substance may occur naked, or surrounded by a thin membrane; but it is always accompanied by a substance called nuclein, which is found in the nucleus. A bit of protoplasm with a nucleus is called a cell. Neither protoplasm nor nucleus can exist if separated, but each soon dies. Some animals consist of only one cell, while others are built up of a countless number of cells. Not only are all animals alike in that they are composed of protoplasm, but all animals take food which they digest and assimilate. All animals are able to throw off waste matter. All animals grow and reproduce.
But the differences between animals are perhaps more striking than the similarities; and were observed much earlier. Thus it has long been known that birds differ from fishes and that both differ from frogs and reptiles. It is very easy to see that horses differ from cows, and both from sheep. Since there are millions of differing animals known, they have been separated into types, or classes, for the sake of convenience. All the animals in each type resemble each other in some important respects, and differ from the animals in other types in these respects. Authorities differ in regard to the number of types which exist, but agree in placing the number between nine and twelve. Animals which are made up of only one cell are called Protozoa, and constitute Type I. Plant-like, fixed animals, made up of many cells, in the form of a modified vase or cylinder with perforated sides, constitute Type II, and are called Porifera. Other types of many-celled animals are the Coelenterata, the Vermes, the Molluscoidea, the Echinodermata, the Annulata, the Arthropoda, the Mollusca, and the Chordata.
| But animals resembling each other sufficiently to be classified together under one type may differ from each other in certain less important respects. Therefore it has been found necessary to divide each type into classes, each class into orders, each order into families, each family into genera, each genus into species. Thus, if the full classification of the domestic cat were to be given, he would be placed under the type Chordata, because he agrees with fishes, frogs, reptiles, birds, and mammals in having a notochord in his early developmental stages. He would furthermore be included in the class Mam-malia, because, like all mammals, the cat possesses mammary or milk glands. He would be placed in the order Carnivora, because he is a flesheater; in the family Felidae, because he possesses retractile claws, has a short face, and teeth differing from other members of the Carnivora. The cat is placed in the genus Felis, because he differs in size and color from other Felidae, like lions, tigers, and panthers. And lastly, he is placed in a different species from the wild cat, because he has changed in habit, size, and color as a result of domestication. In a tabulated form the classification of the domestic cat would appear as follows: | |
| Kingdom | Animalia |
| Type | Chordata |
| Class | Mammalia |
| Order | Carnivora |
| Family | Felidae |
| Genus | Felis |
| Species | Felis domestica |
Every animal is classified according to some such scheme as this.
Animals inhabit the land, the water, or the air. The science which treats of them is called zoology.--ELLEN TORELLE.
See Amoeba; Sponge; Earthworm; Clam; Lamprey; Fish; Birds; Frog; Reptile; Mammals; Insects; Mollusks; and numerous articles on the different animals.
On the intellectual side, alas, the more we come to know of the minds of animals, the more monumental appears the ignorance and stupidity of the best of them. Speaking roughly, the relation of animal ability to human ability is this: If we start with the average successful business or professional man, and from this level drop down through a certain interval of natural capacity, we reach the grade of the ordinary day-laborer. Another drop, equal in amount, brings us to the lowest savages. One more of the same sort, and we reach the level of children of four, of weak-minded men and idiots, of dogs and cats and monkeys and elephants. Still lower comes the general ruck of animals.
It is a hard saying. But what does any actual animal ever do that is beyond the mental power of a human idiot?--E. T. Brewster in McClure's Magazine.
