In the case of the organic and inorganic worlds the proper course obviously is, not to draw conclusions from extreme and highly specialised instances, but to follow life downwards to its simplest and most primitive form, and matter upwards to the form which approaches most nearly to this form of life. Following matter upwards, we find a regular progression from the simple to the complex. Take the diamond, which is one of the simplest of substances, being merely the crystallised form of a single ultimate element, carbon. It is extremely hard and extremely stable. Ascending to compounds of two, three, or more elements, we get substances which are more complex and less stable; and at last we arrive at combinations which involve many elements and are extremely complex. Among these latter substances are some, called colloids, which are neither solid, like crystals, nor fluid, like liquids, but in an intermediate state, like jelly or the white of an egg, in which the molecules have great mobility and are at a considerable distance apart, so that water can penetrate their mass. These colloids are for the most part very complicated compounds of various elements based on a nucleus of carbon, which, from its atom having four poles with strong mutual attractions, is eminently qualified for forming what may be called the inner skeleton of these complex combinations. Colloids of this description form the last stage of the ascending line from inorganic matter to organic life. Next let us trace life downwards towards matter. There is a constant succession from the more to the less complex and differentiated: from man, through mammals, reptiles, fishes, and a long chain of more simple forms, until at its end we come to the two last links, which are the same for all animals, all plants, and all forms of animated existence. The last link but one is the cell, the last of all is protoplasm. Protoplasm, or, as Huxley calls it, ‘the physical basis of life,’ is a colourless jelly-like substance, absolutely homogeneous, without parts or structure, in fact a mere microscopic speck of jelly.