273. There is no part of Man's composite nature, in which the intimate relation between Mind and Body is more obvious, than it is in the formation of habitual modes of activity, whether Psychical or Corporeal; the former, like the latter, being entirely conformable to the Laws which express the ordinary course of the Nutritive operations. A general knowledge of these Laws being therefore essential to the student of Mental Physiology, a concise statement of them will be here given.

274. In the first place, it is characteristic of every living Organism to build itself up according to a certain inherited type or pattern; so that we must attribute to its germ a “formative capacity,” in virtue of which it turns to account both the food and the force which it derives from without, —like the Architect who directs the construction of an edifice, which is raised out of the materials brought together by one set of workmen, by means of the labour furnished by another. But this constructive process may undergo considerable modification under the influence of external conditions; so that the reproduction of the parental model is attended with more or less of variation. This influence is peculiarly manifested in the lower types both of Animal and Vegetable life; many of which display a remarkable polymorphism. Thus the germs of the simpler Fungi, falling upon different kinds of decomposing matter, will develop themselves into forms that differ so strikingly from each other, as to have been accounted not only specifically but generically distinct.