more or less exactness, to the ascending series of now living forms; showing, however, in addition to these, many connecting links between existing classes, which, in the progress of time and development, have diverged widely from each other; while the modern science of embryology as clearly shows that the development of the human being—beginning in a formless, structureless, microscopic speck of protoplasm, comparable in all appreciable respects to the "dawn-animal" of the Palæozoic period, and to the moneron and amœba of to-day—consists in the ascent, step by step, with a good degree of exactness, both of the geological ladder and of the trunk of the animal tree whose branches represent all existing forms of animal life, whose roots are deeply imbedded in the inorganic crust of the earth, and at whose apex appears the genus homo—the crown and consummate flower of organic development. In other words, the individual development of every human embryon is a brief résumé (in which, it is true, some of the chapters are suppressed and others greatly condensed) of the history of the development of animal life on the globe, from its infancy to the present day.
In 1862 Professor Graham pointed out the importance of the two states of matter, described by him as crystalloid and colloid—crystal-like and jelly-like. He says: "The colloidal state is, in fact, a dynamical state of matter; the colloid possesses energy, and may be regarded as the primary source of the forces appearing in the phenomena of vitality."
Although certain colloids have a very simple chemical composition (as silica, for example, which, ordinarily crystalloid, is capable of existing in a colloidal state), the molecular constitution of the colloids in general is undoubtedly highly complex. The molecule of albumen (a typical colloid closely resembling protoplasm), while it consists of but six different chemical elements, is estimated as containing several hundred atoms of these elements, which thus render the molecule an extremely massive one.
Now this massiveness of its molecules confers upon protoplasm a certain mechanical stability favorable to the preservation of organic forms; at the same time endowing it with the chemical instability essential for the constant exchanges of material which constitute nutrition and are characteristic of all living matter.
These massive molecules are also reservoirs of vast amounts of energy of the kind long known as potential—a term which, though likely to vanish, not into thin air but into the thinner ether, is nevertheless a very convenient one. This potential energy, stored up during the slow processes of plant-life and appropriated by animals in the form of food, is liberated or manifested as actual energy in the decomposition of their tissues that is, in the gradual breaking down of tissue-cells, by means of which the animal functions are performed, and carbonic acid, urea, and other excretory compounds are produced.