produce crystals or not, are formed by a symmetrical aggregation of atoms. The latter are arranged in equilibrium in two directions, perpendicular to each other, one parallel to the axis of grouping, and the other at right angles to that axis, so as always to compose a symmetrical figure. The most complicated bodies, so soon as they are brought under the law of definite proportions, and compose chemical species, are made up of molecules in which the atoms are grouped in prisms, in pyramids, in a word, in polyhedra more or less many-sided, but always of perfect regularity; so that, in this case, the differentiation is regulated with marvellous harmony.
We must now rise another degree, and pass from inorganic matter to living matter. What is it that distinguishes the latter from the former? When we make the answer depend on the results of direct experiment, nothing is easier than to establish the differential characteristics of living matter. In the first place, it is organized, that is, the anatomical elements, instead of being homogeneous and symmetrical in all points of their mass, are composed by the association of a certain number of different substances, in which carbon predominates, and which are termed immediate organic principles. Then these elements grow. At no time the same throughout, as to the substance which makes them up, they are in a state of unceasing molecular renewal, of constant metamorphosis, of simultaneous and continuous assimilation and disassimilation. Besides, the various properties these elements may exhibit, contractility, neurility, and so on, are, in consequence of the growing state that characterizes them, in so unstable a condition of equilibrium that the slightest variation in the surrounding medium is enough to occasion some change in the expression of their activity; in other words, they have excessive excitability and irritability. Such, at least, is the region within which physiology is limited; but the fact which it does not clearly enough bring out, yet the thing which is the distinctive mark of life, is the harmonious seeking for each other of all these vital monads, the disposition of biological energies to compose groups of which the end and the reason are found in what we call the individual. The differentiations of inorganic matter occur in molecules that are specific, in whatever bulk they are regarded. The differentiations of living matter take place only in individuals whose build and proportions are strictly determined. An iron bar, an iron crystal, and iron-dust, are all still iron. An organic substance fitted for life is nothing, whenever deprived of connection with an organism. It can display energy, can act; in a word, can be, so far as to be a living substance, only in virtue of taking place and rank in a certain whole, and assuming certain dependencies and connections with other more or less analogous substances. By itself it is not distinguished in essence from dead matter. It is raised to the rank and clothed with the dignity of life only from the time of its reception into that gathering of which the
