_Law of the Constitution of Organisms._—Such is the complex animal. It is organized like the city. But the higher law of this city is that the conditions of the elementary or individual life of all the anatomical citizens are respected, the conditions being the same for all. Food, air, and light must be brought everywhere to each sedentary element; the waste must be carried off in discharges which will free the whole from the inconvenience or the danger of such debris; and that is why we have the different forms of apparatus in the circulatory, respiratory, and excretory economy. The organization of the whole is therefore dominated by the necessities of cellular life. This is expressed in _the law of the constitution of organisms_ formulated by Claude Bernard. The organic edifice is made up of apparatus and organs, which furnish to each anatomical element the necessary conditions and materials for the maintenance of life and the exercise of its activity. We now understand what is the life, and at the same time what is the death, of a complex being. The life of the complex animal, of the metazoon, is of two degrees; at the foundation, the activity proper to each cell, _elementary life_, cellular life; above, the forms of activity resulting from the association of the cells, _the life of the whole_, the sum or rather the complex of elementary partial lives. There is a solidarity between them produced by the nervous system, by the community of the general circulatory, respiratory apparatus, etc., and by the free communication and mixture of the liquids which constitute the media of culture for each cell. We shall have an opportunity of recurring to current ideas as to the morphological constitution of organisms. CHAPTER III. THE CHEMICAL UNITY OF LIVING BEINGS. The varieties and essential unity of the protoplasm—Its affinity for oxygen—The chemical composition of protoplasm—Its characteristic substances.—§ 1. The different categories of albuminoid substances—Nucleo-proteids—Albumins and histones—Nucleins.—§ 2. Constitution of nucleins.—§ 3. Constitution of histones and albumins—Schultzenberger’s analysis of albumin—Kossol’s analysis—The hexonic nucleus. The chemical unity of living beings corresponds to their morphological unity.