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.
Value of Form as a Characteristic of Living and Brute Beings.—However this may be, we may say, without fear of exaggeration, that the crystalline form characterizes the mineral with no less precision than the anatomical form characterizes the animal and the plant. In both cases, form—regarded as a method of distribution of the parts—indicates the individual and allows us to diagnose it with more or less facility.
Parentage of Living Beings and Mineral Parentage.—Still another analogy has been noted. In animals and plants similarity in form indicates similarity in descent, community of origin, and proximity in any scheme of classification. In the same way identity of crystalline form indicates mineral relationship. Substances chemically analogous show identical, geometrically superposable forms, and are thus arranged in family or generic groups recognizable at a glance.
Isomorphism and the Faculty of Cross-breeding.—And further, the possibility in the case of isomorphous bodies, of their replacing each other in the same crystal during the process of formation and of thus mingling, so to speak, their congenital elements, may be compared with the possibility of inter-breeding with living beings of the same species. Isomorphism is thus a kind of faculty of crossing. And as the impossibility of crossing is the touchstone of taxonomic relationship, testing it, and separating stocks that ought to be separated, so the operation of crystallization is also a means of separating from an accidental mixture of mineral species the pure forms which are blended therein. Crystallization is the touchstone of the specific purity of minerals; it is the great process in chemical purification.
Other Analogies.—The analogies between crystalline and living forms have been pushed still further even to the verge of exaggeration.
The internal and external symmetry of animals and plants has been compared to that of crystals. Transitions or intergradations have been sought between the rigid and faceted architecture of the latter and the flexible structure and curved surface of the former; the utricular form of flowers of sulphur on the one hand, and the geometrical structure of the shells of radiolarians on the other, have shown an exchange of typical forms between the two systems. An effort has even been made to draw a parallel between six of the principal types of the animal kingdom and the six crystalline systems. If carried as far as this, our thesis becomes puerile. Real analogies will suffice. Among these the curious facts of crystalline renewal come first.