The next grade of structural complication in living creatures is produced by the lowly plants, such as _Protococcus_, which multiply by spontaneous self-division or _fission_. This process may take place repeatedly and at the same time incompletely, in this way producing an apparently compound organism. Thus, we have the second grade of structural complication in living creatures--namely, the aggregation of cells into a loosely joined mass. Other simple forms are those presented by the minute organisms Diatoms and Desmids, the former enclosed in silicious cases, and some presenting the only exception to the general law that organic bodies are bounded by curved lines and surfaces. Wonderful is the minute ornamentation presented by the surfaces of these microscopic plants. Some of them cohere by imperfect division in the second grade of structural complication just described; they may form longitudinal series of cells, or they may be arranged round a common centre. One of the best examples of this secondary grade of complication is presented by the spherically aggregated cells of _Volvox_.[91] These present us with a good example of the way in which the shape of the individual cells may spontaneously alter, to suit the mode of their aggregation. Originally spherical, the adjacent sides of these cells become flattened, and thus the cells acquire a polygonal figure. Other instances of the coherence of the cells of unicellular organisms into indefinite and inconstant aggregations is presented by some radiolarians, individuals which cohere into what are called _colonies_. From such incomplete aggregation, the next step is to definite and stable aggregations, in which the life of the constituent parts is more or less plainly subservient to, and dominated by, the life of the whole. Such we find in all but the lowest _Fungi_,[92] and _Algæ_, in sponges,[93] and _Hydræ_, and also in all higher organisms. In such permanent aggregations, the dominant life of the whole is shown partly in greater constancy of external form and partly in the setting apart of separate portions of the whole, either for the nourishment of the entire creature or for the reproduction of fresh individuals, or for effecting gaseous interchange, or (in animals) for ministering to feeling and locomotion.