But are we entitled from these facts to indulge in self-glorification, and to assert that ultimately the exact reproduction of all the tissues found in living organisms will be possible? Certainly not, so far as the chemist is concerned, for other methods than those which properly belong to his science must obviously be sought to give the specialized forms and functions of living substance. The gulf which separates the artificial processes of synthetic chemistry from those really employed by the plant or animal is a wide one, across which lies as yet no solid bridge of fact and theory. It is known that all vegetable matter is derived primarily from carbonic acid, water, and ammonia, under the influence of the sun's rays, but the method by which this is done is still a mystery; a mystery, though, which may be cleared away, for very recently most interesting experiments have been made on the chemical action of light. All we know about vegetable chemistry with certainty is, that, whatever are the processes occurring in living tissue, most of them, in their earlier stages, are radically different from those of art. In the laboratory, synthesis starts with the elements, and from them, by exceedingly wasteful means, builds laboriously, from platform to platform, up to the desired height. On the other hand, the smallest speck of green vegetable tissue, if living, elaborates its substance, not from elementary bodies, but from compound ones, and those too which are among the most stable and most highly-oxidized known; each new step, then, in the artificial method, tends to carry us away, rather than to approximate us to the natural agent.
To return to the metaphor of the house. The chemist starts from the ground, and completes the edifice by piling up one by one, the elementary bricks, and binding them together by the natural cement of atomic attraction. The living organism begins its labors at the top, and chiefly from the three firmly-knit compounds, water, carbonic acid, and ammonia, builds, by infinite gradation, down to an elementary foundation; and, at this day, science is scarcely more able to tell how this is done, than can the mason inform us how to start a block of houses by commencing with the roof.
This distinction as to methods includes obviously a vast domain of facts, that which has been referred to as the region lying between the natural and artificial synthetic methods. It is a sort of debatable land, for, though it is yet unknown, there is not the slightest proof that we shall not cross it some day, and possibly soon; and already the space is inhabited by speculations and embryonic theories, those shadowy precursors of substantial knowledge.
But the limit where the chemist must stop is the dividing line where the naked individuality of a chemical compound becomes clothed by the definite outline of an organ, a cell, or a speck of vitalized protoplasm; as soon as form other than crystalline appears, the sharpness of atomic characteristics is merged in the idiosyncrasies of the cellular