first signs it gives of its activity are movements comparable with those of the amoeba. Thus, without effort, we find on different sides life freed from form. We comprehend that it is not essentially and fatally bound to form. A body may be living and still have no definite figure. Here the problem is suggested, whether a liquid, a bodily humor, can be living. Is the blood living, like the substance of the nerves or the flesh of the muscles? It is a deep question and has not yet been answered. At any rate, science has been led for a long time to look for the characteristic of life somewhere else than in form.
The Aristotelians saw a movement in what we call life; and they gave that name to every change of state of natural bodies as well as to their translation proper in space. Aristotle's treatise on the Soul characterizes life by the three facts of its nourishing itself, developing, and perishing. Growth and decline are changes, and consequently movements; and, as we always see them closely connected with the feeding of the plant as well as of the animal, we find the act of feeding definitively at the basis of the movement which is life. Moreover, do we not see during growth the parts of which the creatures are composed changing places relatively to one another? Have we not here a clear, absolute distinction from the increase of mineral bodies?
There are, however, some parts in animals which grow by a simple constant accretion of superadded new particles; such as the shells of mollusks, even when they are covered by the flesh, like cuttlefish bone. But these forinations, although derived from the organism, are not themselves living. They bear, if we may say so, the stamp and seal of life so far that we can recognize them as a product of it, but no further; and if they grow, it is as crystals do.
Thomas Aquinas, following Aristotle, gave life the most exact definition that could be made with the knowledge of his time. It is almost as satisfactory for us, for we, too, define life in the same terms. It is a movement, but still not one of the apparent though intimate movements to which the Christian encyclopedist alludes. It is a molecular movement that escapes our eyes, in the interior of the being, and is revealed to our senses only by its results.
The movement that constitutes life is an intimate, profound, invisible, incessant movement, at once of combination and of decomposition. Living matter is incessantly born and incessantly dying, being formed and suffering destruction all at the same time.
All liquid or gaseous bodies coming in contact with a living substance and soluble by it, penetrate it, mingle with it, and then, carried on in the whirl, cease for the most part to be themselves, are transformed, enter into new combinations that did not exist
