a less stable to a more stable equilibrium. The atoms of the organic substance lose part of their latent motion, which is manifested externally under the form of heat, electricity, light, nervous force, or mechanical motion, according to circumstances. Be the cause which produces these changes necessary or not, they are, of necessity, accompanied by a disengagement of force; and we can affirm of any force whatever expended by an organ of a living being, that it is the equivalent of a force acting from without upon that being. This is a consequence of the law of the persistence of force, and it may be presented under two forms: First, in order that a certain amount of force may be expended by a living being, there must have taken place, within that being, a transformation, by decomposition, of a quantity of organic substance capable of holding that force in the latent stale; and, secondly, there can be no transformation of organic matter holding force in the latent state, without an expenditure of force which shall manifest itself in some shape externally.
In general terms, what we have to consider in living things is, first, a substance of special composition, and then expenditures of force by that substance; and this, too, is what we have in general terms to consider, in non-living things. The former are distinguished from the latter by the fact that the changes which constitute their history are heterogeneous; that they form many series which are simultaneous, correlative, held together by a tie of mutual dependence, the result being a high degree of complexity, a phenomenon belonging to one series having antecedents and consequents in other simultaneous series; and above all, that these changes form clearly-defined combinations. This ensemble of characteristics not only enables us to distinguish living from non-living things, but also to distinguish between living things themselves and to class them according to their degree of life. Thus a thing stands all the higher in the vital scale in proportion as, from the beginning to the end of its vital manifestations, it exhibits a larger number of successive and simultaneous changes, and as these changes are more heterogeneous and more closely linked together, and in more definite relations to one another. Between the lowest animals, rhizopods, planaria, etc., and the highest, the birds of prey, manmalia, carnivora, man, there is an enormous dissimilarity; still the definition applies to them all, and serves to define the difference which separates them, as also the difference of the numerous species lying between these extremes of the animal series.
Though this definition is a good one, inasmuch as it applies to all living things, and to them alone, nevertheless it is defective in that it omits the most distinctive peculiarity, namely, the element known as activity, in other words, those operations whereby living beings adapt themselves to their conditions of life. The definition should include the general relations of the living thing to its environment. The environment, too, has its successive and correlative changes
