of the chemist, where organic matter goes through a series of gradual modifications by which it is adapted to new artificial conditions; and, on the other hand, facts observed in the lowest orders of animals by the biologist. We conceive that, in the primordial world, as now in the laboratory, higher types of organic substance were formed at the expense of lower types, and that gradually, after repeated reactions and under favorable conditions, they resulted in organizable protoplasm, a substance which is very susceptible of modification. Protein, as we know, may exist in upward of one thousand isomeric forms, and, by combination with itself and with other substances, it yields products still more complex, and in countless numbers. Hence we can easily conceive how, under the conditions of heat and light then existing on earth, and with the aqueous, mineral, and atmospheric environment of that epoch, protein may have undergone metamorphoses without end. Under conditions which we can conceive as possible, though we may not be able to define them exactly, products may have been evolved fitted to exhibit the rudimentary vital reactions. In this way we fill up the chasm which divides the positive chemical facts of the higher organic combinations from the biological phenomena of the lower forms of life.
But another hypothesis is still necessary. "When we come down to the substances out of which living bodies are formed, we find groups and sub-groups of manifold and divergent compounds, the units of which are large, heterogeneous, and unstable in a high degree. Why should we suppose that these combinations must stand still at the complex colloids which enter into the composition of organic matter? Is it not more probable that, in addition to these colloids, there are developed by a higher combination atoms still more heterogeneous and compounds still more numerous? If colloids are unstable, extremely modifiable by very slight incident forces, and incapable of assuming the equilibrated form of crystallization, then a fortiori these new organic atoms are unstable, very modifiable, and of many different species." They would surpass protein in instability and plasticity as much as protein surpasses organic matter. Furthermore, these atoms would possess one fundamental property, without which no explanation is possible in biology, viz., the property of arranging themselves in certain forms peculiar to the various groups to which they belong—a property but little understood, though its existence is unquestionable. We call it polarity, for want of a better term, to indicate the power of manifesting actions in a certain fixed direction. These atoms we denominate physiological units. They are developed in every living thing, differentiating themselves from one another in different organisms by the same causes which differentiate the organisms themselves, and in this way acquiring a diversity which corresponds to that of the creature they constitute by their aggregation. They follow, step by step, in their modifications the modi-
