changed into living substance, which, after serving its purpose, passes again as waste to the inorganic world.

(2) Structure. Minerals are homogeneous, while organized bodies are usually heterogeneous, i.e., composed of different parts, called tissues and organs, having peculiar uses and definite relations to one another. The tissues and organs, again, are heterogeneous, consisting mainly of microscopic cells, structures developed only by vital action. All the parts of an organism are mutually dependent, and reciprocally means and ends, while each part of a mineral exists for itself. The smallest fragment of marble is as much marble as a mountain mass; but the fragment of a plant or animal is not an individual.

(3) Shape and Size. — Living bodies gradually acquire determinate dimensions; so do minerals in their perfect or crystal condition. But uncrystallized, inorganic bodies have an indefinite bulk. Most minerals are amorphous; crystals have regular forms, bounded, as a rule, by plane surfaces and straight lines; plants and animals are circumscribed by curved surfaces, and rarely assume accurate geometrical forms. 63

(4) Phenomena.- Minerals remain internally at rest, and increase by external additions, if they grow at all. Living beings are constantly changing the matter of which they are composed, and grow by taking new matter into themselves and placing it among the particles already present. Organized bodies, moreover, pass through a cycle of changes, growth, development, reproduction, and death. These phenomena are characteristic of living as opposed to inorganic bodies. All living bodies grow from within, constantly give up old matter and replace it by new, reproduce their kind, and die; and no inorganic body shows any of these phenomena.

CHAPTER V*

PLANTS AND ANIMALS DISTINGUISHED

It may seem an easy matter to draw a line between plants and animals. Who cannot tell a cow from a cabbage? Who would confound a coral with a mushroom? Yet it is impossible to assign any absolute, distinctive character which will divide the one form of life from the other. The difficulty of defining an animal increases with our knowledge of its nature. Linnæus defined it in three words;† a century later, Owen declared that a definition of plants which would exclude all animals, or of animals which would not let in a single plant, was impossible. Each different character used in drawing the boundary will bisect the debatable ground in a different latitude of the organic world. Between the higher animals and higher plants the difference is apparent; but when we reflect how many characters the two have in common, and especially when we descend. to the lower and minuter forms, we discover that the two "kingdoms" touch, and even dissolve into, each other. This border land has been as hotly contested among naturalists as many a disputed frontier between adjacent nations. Its inhabitants have been taken and retaken several times by botanists and zoologists; for they have characters that lead on the one side to plants, and on the other to animals. To solve the difficulty, some eminent naturalists, as Hæckel and

* See Appendix.

+"Minerals grow; plants grow and live; animals grow, live, and

feel."

Owen, propose a fourth "kingdom," that of the Protista, to receive those living beings which are organic, but not distinctly vegetable or animal. But a greater difficulty arises in attempting to fix its precise limits.

The drift of modern research points to this: that there are but two kingdoms of nature, the mineral and the organized, and these closely linked together; that the latter must be taken as one whole, from which two great branches rise and diverge. "There is at bottom but one life, which is the whole life of some creatures and the common basis of the life of all; a life of simplest moving and feeling, of feeding and breathing, of producing its kind and lasting its day: a life which, so far as we at present know, has no need of such parts as we call organs. Upon this general foundation are built up the manifold special characters of animal and vegetable existence; but the tendency, the endeavor, so to speak, of the plant is one, of the animal is another, and the unlikeness between them widens the higher the building is carried up. As we pass along the series of either [branch] from low to high, the plant becomes more vegetative, the animal more animal." 64

Defining animals and plants by their prominent characteristics, we may say that a living being which has cell walls of cellulose, and by deoxidation and synthesis of its simple food stuffs produces the complicated organic substances, is a plant; while a living being which has albuminous tissues, and by oxidation and analysis reduces its complicated food stuffs to a simpler form, is an animal. But both definitions are defective, including too many forms, and excluding forms that properly belong to the respective kingdoms. No definition is possible which shall include all animals and exclude all plants, or vice versa.

(1) Origin. - Both branches of the tree of life start alike the lowest of plants and animals consist of a single cell. In fact, the cycle of life in all living beings begins in a small, round particle of matter, a cell—in the higher plants called an ovule, in the higher animals an ovum. This cell consists mainly of a semifluid substance called protoplasm. In the very simplest forms the protoplasm is not inclosed by a membrane or cell wall. In most plants the cell wall is present, and consists of cellulose, a substance akin to starch; in animals, with few exceptions, the wall is a pellicle of firmer protoplasm, i.e., albuminous.

(2) Composition. Modern research has broken down the partition between plants and animals, so far as chemical nature is concerned. The vegetable fabric and secretions may be ternary or binary compounds; but the essential living parts of plants, as of animals, are quaternary, consisting of four elements, — carbon, hydrogen, oxygen, and nitrogen. Cellulose (woody fiber), starch, and chlorophyl (green coloring matter) are eminently vegetable products, but not distinctive; for cellulose is wanting in some plants, as some fungi, and present in some animals, as tunicates; starch, under the name of glycogen, is found in the liver and brains of mammals, and chlorophyl gives color to the freshwater polyp. Still, it holds good, generally, that plants consist mainly of cellulose, dextrin, and starch; while animals are mainly made up of albumen, fibrin, and gelatin; that nitrogen is more abundant in animal tissues, while in plants carbon is predominant.

(3) Form. -No outline can be drawn which shall be common to all animals or all plants. The lowest members of each group have no fixed shape. The spores of Confervæ can hardly be distinguished from animalcules; the compound and fixed animals, sea mat and

sea moss (Polyzoa), and corals, often resemble vegetable forms, although in structure widely removed from plants. Similar conditions of life are here accompanied by an external likeness. In free-living animals this resemblance is not found.

(4) Structure. A plant is the multiplication of the unit — a cell with a cellulose wall. Some simple animals have a similar simple cellular structure; and all animal tissues, while forming, are cellular. But this character, which is permanent in plants, is generally transitory in animals. In the more highly organized tissues the cells are so united as partly or wholly to lose their individuality, and the characteristic part of the tissue is the intercellular substance, while the cells themselves are small and unimportant, or else the cells are fused together and their dividing walls become indistinct, as in glandular tissue. Excepting the lowest forms, animals are more composite than plants, ie., their organs are more complex and numerous, and more specially devoted to particular purposes. Repetition of similar parts is a characteristic of plants; and when found in animals, as the angleworm, is called vegetative repetition. Differentiation and specialization are characteristic of animals. Most animals, moreover, have fore-and-aft polarity; in contrast, plants are upand-down structures, though in this respect they are imitated by radiate animals, like the starfish. Plants are continually receiving additional members; most animals soon become perfect.

(5) Physiology. In their modes of nutrition, plants and animals stand widest apart. A plant in the seed and an animal in the egg exist in similar conditions: in both cases a mass of organic matter accompanies the germ. When this supply of food is exhausted, both seek nourishment from without. But here analogy