_Conclusion. Relations of the Surrounding Medium to the Living Being and the Brute Body._—Brute bodies, then, are not immutable any more than are living bodies. Both depend on the medium that surrounds them, and they depend upon it in the same way. Life brings together, brings into conflict, an appropriate organism and a suitable environment. Auguste Comte and Claude Bernard have taught us that vital phenomena result from the reciprocal action of these two factors which are in close correlation. It is also from the reciprocal action of the environment and the brute body that inevitably result the phenomena which that body presents. The living body is sometimes more sensitive to variations of the ambient medium than is the brute body, but at other times the reverse is the case. For example, there is no living organism as impressionable to any kind of stimulus whatever as the bolometer is to the slightest variations of temperature. There can only be, then, one chemically immutable body—namely, the atom of a simple body, since, by its very definition, it remains unaltered and intangible in combinations. This notion of an unalterable atom has, however, itself been attacked by the doctrine of the ionization of particles due to Sir J. J. Thomson; and besides, with very few exceptions—those of cadmium, mercury, and the gases of the argon series—the atoms of simple bodies cannot exist in a free state. Thus, as in the vital struggle, the ambient medium by means of alimentation furnishes to the living being, whether whole or fragmentary, the materials of its organization and the energies which it brings into play. It also furnishes to brute bodies their materials and their energies. It is also said that the ambient medium furnishes to the living being a third class of things, the _stimuli_ of its activities—_i.e._, its “provocation to action.” The protozoon finds in the aquatic environment which is its habitat the stimuli which provoke it to move and to absorb its food. The cells of the metazoon encounter in the same way in the lymph, the blood, and the interstitial liquids which bathe them, the shock, the stimulus which brings their energies into play. They do not derive from themselves, by a mysterious spontaneity without parallel in the rest of nature, the capricious principle which sets them in motion. Vital spontaneity, so readily accepted by persons ignorant of biology, is disproved by the whole history of the science. Every vital manifestation is a response to a stimulus, a provoked phenomenon. It is unnecessary to say this is also the case with brute bodies, since that is precisely the foundation of the great principle of the inertia of matter. It is plain that it is also as applicable to living as to inanimate matter. CHAPTER V. SPECIFIC FORM. LIVING BODIES AND CRYSTALS.