An organism consists of two great groups of _structural units_ called _cells_,—=Germ-Cells= and =Body-Cells=,—harmoniously associated together. As far as Evolution is concerned, the most fundamental parts of an organism are the Germ-Cells. In order that the reader may have a working knowledge of the wonderful powers of Germ-Cells and their progeny, it is vitally important that some concrete illustrations of the activities of different kinds of cells should be given. SECTION I. ORGANIC CELLS: THE VISIBLE UNITS OF LIFE. ORGANIC CELLS: THE VISIBLE UNITS OF LIFE. In the sanctuary of S. Vitale at Ravenna, in Italy, is a very interesting representation, in mosaic pictures, and over life-size, of the Emperor Justinian and his Empress Theodora, attended by a numerous suite of ladies and courtiers. The mosaics are small bits of glass, of varying pattern and color, cemented together so nicely as to form beautiful delineations of the Emperor and his attendants. The bits of glass or mosaics that form the figures may very appropriately be called _structural units_. The body of man, a bird, a lizard, an oak tree, and many other animals and plants, may usefully be compared to such mosaic figures; for, just as the mosaic figure has its structural unit, the little bit of glass or stone, called the mosaic; so the bodies of men, birds, and other creatures may be looked upon as infinitely complex figures formed of minute mosaics called _cells_. The groups of minute mosaics or _cells_ that make up the bodies of animals and plants differ profoundly from the mosaics that form the figures of the dead Emperor and his companions, inasmuch as each mosaic or cell of the animal or plant body is so small as to require the microscope to reveal it; also each mosaic of the living animal or plant is a living mosaic or cell, in that it can absorb food, digest it, assimilate it, grow, and multiply in numbers. Cells, then, are the _morphological or structural units_ which compose the bodies of all living creatures. All animals and plants begin life as _single_ cells, which, in the vast majority of cases, are microscopic in size. Those that remain single cells, and dissociated throughout life, are called _Unicellular_ Animals (Protozoa) and Plants (Protophyta), and are to be seen mostly by the microscope alone; but those which, by multiplication and growth, form large numbers of cells that remain associated together as in the body of a bird or lizard, are called _Multicellular_ Animals (Metazoa) or Plants (Metaphyta).