Constructing theories and speculations as to the possibilities of spontaneous generation is regarded by some naturalists as somewhat gratuitous (_cf._ Du Bois-Reymond). In general, it is regarded as sufficient to point out that the reduction of the phenomena of life as we know them to those of a simpler order, and the unification of organic and inorganic chemistry, have made the problem of the first origin of life essentially simpler, and that the law of the constancy and identity of energy throughout the universe permits no other theory. But others go more determinedly to work, and attempt to give concrete illustrations of the problem. The most elementary form of life known to us is the cell. From cells and their combinations, their products and secretions, all organisms, plant and animal alike, are built up. If we succeed in deriving the cell, the derivation of the whole world of life seems, with the help of the doctrine of descent, a comparatively simple matter. The cell itself seems to stand nearer to the inorganic, and to be less absolutely apart from the inanimate world than a highly organised body, differentiated as to its functions and organs, such as a mammal. It almost seems as if we might regard the lowest forms of life known to us, which seem little more than aggregated homogeneous masses of flowing rather than creeping protoplasm, as an intermediate link between the higher forms of life and the non-living. But the theory does not begin with the cell; it assumes a series of connecting-links (which may of course be as long and as complicated as the series from the cell upwards to man) between the cell and matter which is still quite “inorganic” and which is capable only of the everyday chemical and physical phenomena, and not of the higher syntheses of these, which in their increasing complexity and diversity ultimately come to represent “life” in its most primitive forms. As proteid is the chief constituent of protoplasm, it is regarded as the specific physical basis of life, and life is looked upon as the sum of its functions. And it is not doubted that, if the conditions of the universe brought about a natural combination of carbon, hydrogen, nitrogen and oxygen in certain proportions, so that proteid resulted, the transition to proteid which forms itself and renews itself from the surrounding elements, to assimilating, growing, dividing proteid, and ultimately to the most primitive plasmic structure, to non-nucleated, nucleated, and finally fully formed cells, could also come about.