Photo: F. Martin Duncan.] The life-history of an ordinary one-celled organism may be briefly summed up. It absorbs nourishment and energy, adds to its substance until it reaches a certain fairly definite size, and then divides in two, the halves separating, and going each its own way. In the world of one-celled organisms there is no 'death from natural causes.' The individual is potentially immortal, except in so far as we may regard the individual life as ceasing when division takes place. Death occurs only, as we say, accidentally--for example, from starvation or from the attacks of enemies. A number of simple unicellular organisms are shown in Figs. 18, 19, and 20. The reader will have observed that we have referred to the group under consideration in general terms, and without endeavouring to classify its members as plants or animals. And indeed it is impossible to carry this great distinction down to the lowest group of the organic world. This stands below the first great forking of the tree of life; its members remain in what has been described as a condition of 'chronic indecision,' neither clearly vegetable nor definitely animal. But very soon, in the march of progress, the forking of the roads was reached, and whosoever was bent on journeying farther had perforce to make the choice. We must here briefly consider what this choice was, and wherein the fundamental distinction between a plant and an animal consists; for, strange as the statement may seem, the basis of this distinction is by no means generally appreciated. [Illustration: FIG. 19.--The bacillus of bubonic plague (× 1000). Photo: F. Martin Duncan.] The typical plant lives by absorbing carbon dioxide gas, water, and mineral salts from the surrounding media. These substances, by means of energy which it gathers from the rays of the sun, the plant builds up into organic substances, to be used in the maintenance of life, and for growth and reproduction. This process of chemical construction occurs only in the green, exposed parts of the plant, and indeed can occur only in the presence of chlorophyll, the green colouring matter of the leaves. [Illustration: FIG. 20.--The bacillus of typhoid (× 2500 diameters). Photo: F. Martin Duncan.]