To begin at the beginning of our tale, we may ask ourselves what are the lowest, simplest, living things that are known. The question does not admit of any very definite answer. For as we look around among a number of the most simple forms, we find ourselves handicapped in our attempt to judge between them, by a lack of knowledge of their nature. We come upon organisms so small that they appear, even under the most powerful microscope, only as the tiniest specks; whose size is to be measured in hundredths of thousandths of an inch. We even find good evidence that living things exist which we are unable, in any manner whatsoever, to see. Among the smallest known forms, and also among some of the larger, we find organisms that we can only describe as practically structureless, that appear as specks of almost homogeneous protoplasm; but it seems reasonable to suppose that this appearance is due rather to our imperfect observation than to an actual absence of differentiation. It is certain, however, that the lowest of the great groups is that of the one-celled organisms. As all the higher types are built up of large numbers of cells, essentially similar to those which constitute the unicellular forms, it is important that we should know something of the nature of this organic unit. A typical cell is illustrated in Fig. 16. It consists of a mass of protoplasm, with a distinctly differentiated portion called the nucleus. The function of the nucleus is that of directing and controlling the activities of the cell; if it is removed, the remaining portion of the cell soon dies; while, on the other hand, a small portion of the cell, if it contains the nucleus, may frequently live, and build up new protoplasm to replace what was lost. Cells are formed only from previously existing cells, by a process of division, which is usually simply one of halving. This process is begun in the nucleus; it undergoes a complex rearrangement of its parts, the object of which appears to be to insure an absolute equality in the halves, and finally divides in two. The bulk of the protoplasm then separates into two portions, a portion remaining round each of the nuclei. The process of cell division is illustrated in Fig. 17. Now it is a somewhat remarkable fact that we do not know whether or not all the humbler forms of life possess a nucleus. It was formerly believed that a considerable number of one-celled organisms were devoid of the body in question, but in most of such it has been shown that nuclear matter is present, though it may be distributed, in small portions, throughout the cell. If organisms do exist which consist of a cell without a nucleus, we must regard them as the simplest of living things. In any case, the formation of a nucleus, a process by which a kind of central government was formed, was probably one of the great early steps of Evolution. [Illustration: FIG. 18.--Organism of sleeping sickness in blood. The round bodies are red blood corpuscles.