These very general outlines will suffice to show how complex and adjustable is the organization of the crystalline individual, which in spite of its geometric regularity and its rigidity, may be compared with the still more flexible organization of the living element. The mineral individual is more stable, more labile—_i.e._, less prone to undergo change than is the living individual. We may say with M. Lapparent that “crystallized matter presents the most perfect and stable orderly arrangement of which the particles of bodies are susceptible.” _Law of Relation of Specific Form to Chemical Constitution._—Crystallization is a method of acquiring specific form. The geometrical architecture of the mineral individual is but little less wonderful or characteristic than that of the living individual. Its form is the result of the mutual reactions of its substances and of the medium in which it is produced; it is the condition of material equilibrium corresponding to a given situation. This idea of a specific form belonging to a given substance under given conditions must be borne in mind. We may consider it as a kind of principle of nature, an elementary law, which may serve as a point of departure for the explanation of phenomena. A particular substance under identical conditions of environment, must always assume a certain form. This close linking of substance and form, admitted as a postulate in physical sciences, has been carried into biology by some philosophical naturalists, by M. Le Dantec, for instance. Let us imitate them for a moment. Let us cease to seek in the living being for the prototype of the crystal; let us, on the contrary, seek in the crystal the prototype of the living being. If we succeed in this, we shall then have found the physical basis of life. Let us say, then, with the biologists we have mentioned, that the substance of each living being is peculiar to it; that it is specific, and that its form—that is to say its organization—follows from it. The morpholpgy of any being whatever, of an animal—of a setter, for example—or even of a determinate being—of Peter, of Paul—is the “crystalline form of their living matter.” It is the only form of equilibrium that can be assumed under the given conditions by the substance of the setter, of Peter, or of Paul, just as the cube is the crystalline form of sea-salt. In this manner these biologists have supposed that they could carry back the problem of living form to the problem of living substance, and at the same time reduce the biological mystery to the physical mystery. I have shown above (Chap. V. pp. 199-204) how far this idea is legitimate, and how far and with what restrictions it may be welcomed and adopted.