Water is a constituent element of the organisms. They contain fixed proportions for the same tissue, proportions varying from one tissue to another (between 2∕3 and 9∕10). The cell of a living tissue requires around it an aqueous atmosphere, formed by the different juices of the organism, the interstitial liquids, the blood, and the lymph. We are deceived by appearances when we distinguish between aerial, aquatic, and land-dwelling animals, and when we speak of the air, the water, and the land as their natural environment. If we go to the bottom of things, and fix our attention on the real living unities, on the cells of which the organism is composed, we shall find around them the juices, rich in water, which are their real environment. If these juices are diluted or concentrated the least in the world, life stops. The cell, the whole animal, falls into a state of latent life, or dies. “All living beings are aquatic,” said Claude Bernard. “Beings that live in the air are in reality wandering aquariums,” said another physiologist. “No moisture, no life,” wrote Preyer. The environment must contain water, but it must contain it in certain proportions. In the higher animals there is a mechanism which works automatically to keep at a constant level the quantity of water in the blood. Researches on the lavage of the blood (A. Dastre and Loye) have clearly shown this. Oxygen is also necessary to life. It is the _pabulum vitæ_. But the discovery of the beings called by Pasteur _anaerobia_ appears to contradict this statement. Pfeffer, the illustrious botanist, was certain, in 1897, that the dogma of the necessity of oxygen no longer held good. This is no longer tenable. In 1898 Beijerinck carried out most careful researches on anaerobia said to have been cultivated in a vacuum, such as the _bacteria of tetanus_ and the _septic vibrion_; or on those to which oxygen seems to be a poison, such as the _butyric_ and the _butylic ferments_, the anaerobia of putrefaction, the reducing spirilla of the sulphates. All use free oxygen. They consume very little it is true; they are micro-aerobia. The other organisms, on the contrary, need more. They are macro-aerobia or simply _aerobia_. Besides, if the so-called anaerobia take little or no free oxygen, it matters little. They take the oxygen in combination. It may be said with L. Errera that they have an affinity for oxygen, for they extract it from its combinations, and that “they are so well adapted to this mode of existence that life in the open air being too easy no longer suits them.” There are for the different animal species different optima of oxygen.