and in what comes hereafter we must to a great extent follow his lead. The algae which are found in animal substance have been referred to above as "parasitic," but it is chiefly to avoid the use of this term that the more accurate one (symbiosis) has been employed. A closer analogy than that offered by the lichens would be, it seems to me, afforded by any perfect plant—an oak, for example. Here the colorless cells—of the root, let us say—are bound to live at the expense of the green cells in the stem and leaves. Yet we do not think of this as a parasitic event. The root-cell is rather a unit in a vast colony of units (cells) associated for mutual benefit. The green cell gets quite as much good from the root-cell as the latter gets from the green cell; water and salts are exchanged by the root-cells for sugary matters and other things readily made use of by any cell, and no harm (as would be the case in parasitism), but rather much good, is done by the exchange. Evidently the oxygen thrown off by the alga is precisely what the plant needs, and the carbonic acid and nitrogenous waste eliminated by the animal is most useful to the alga. Moreover, the algae gain the advantage of ready locomotion with their host, and the animal can go further into unfavorable media when stocked with algae ready to build up starches and sugary matters from carbonic acid and water. If either dies, the other is the gainer; since the algae can thrive on the products of animal decomposition, and algæ—digestible, i. e., dead algæ—are much esteemed by most animals.
The whole burden of the physiological history of symbiosis is forcibly summed up by Mr. Geddes as follows: "Thus, then, for a vegetable cell no more ideal existence can be imagined than that within the body of an animal cell of sufficient vital activity to manure it with carbonic acid and nitrogen waste, yet of sufficient transparency to allow the free entrance of the necessary light. And, conversely, for an animal cell there can be no more ideal existence than to contain a vegetable cell, constantly removing its waste products, supplying it with oxygen and starch, and being digestible after death. . . . In short, we have here the relation of the animal and the vegetable world reduced to the simplest and closest conceivable form.
"It must be by this time sufficiently obvious that this remarkable association of plant and animal is by no means to be termed a case of parasitism. If so, the animals so infested would be weakened, whereas their exceptional success in the struggle for existence is evident. Anthea cereus, which contains most algae, probably far outnumbers all the other species of sea-anemones put together, and the radiolarians, which contain yellow cells, are far more abundant than those which are destitute of them. . . . Such an association is far more complex than that of the fungus and alga in the lichen, and indeed stands unique in physiology as the highest development, not of parasitism, but of the reciprocity between the animal and vegetable kingdoms."