manent change of structures and functions; that a disturbing influence, even though it were to extend to many generations, can only modify a race superficially; and, finally, that, the instant that this cause ceases to be, the race resumes, slowly but surely, its original characters.
In fact, the environment is ever changing, and in the enormous cycles of changes in the conditions surrounding organic life upon the earth the same conditions have never occurred a second time. Organisms must follow this movement of variation; they must be ever undergoing a process of adaptation, in order to be in equilibrium with the altered conditions around them. In this necessity for adaptation we recognize a consequence of our first principles. The state of homogeneity must give way to a state of heterogeneity: a species must be ever growing more and more varied in its forms; old species must be ever breaking up into new. If at one time a species consisted of individuals alike in all respects, the action of the various forces of the environment would soon put an end to this uniformity; at the same time, however, leaving tokens of relationship. But let us go further, and suppose the conditions to be still more profoundly altered, owing, for instance, to a climatic perturbation of the habitat, or to an emigration of the species into other habitats; in that case there will be different sets of conditions, and the groups of individuals will resemble one another, or be unlike, according to the likeness or unlikeness of the conditions. The connection between changes in the conditions, changes in function, and changes in structure, is a consequence of the persistence of force.
The law of heredity, which is antagonistic to the law of variation, may also be traced back to our first principles. This law represents the element of fixity in the domain of life. All the organisms of a given type are descended from organisms of the same type. If we consider heredity in a succession of organisms, it appears to be inexplicable. Many still deny the existence of heredity, and explain the resemblance of the child to its parentage by a special intervention of the creative power of Nature. But, if we compare the heredity of the individual with certain phenomena occurring in the individual, for example, the repair of tissues, the reproduction of worn-out or lost parts—a process which in some animals goes so far as to reproduce highly-complex organs or groups of organs (for instance, in lizards, the reproduction of feet and tail; the reconstruction of the fresh-water hydra; the restoration of the plant Begonia Phyllomaniaca from a fragment of its leaf)—we shall perceive that there exists a tendency to reproduce like products, and that the two orders of phenomena are related. We must suppose them both to be due to the tendency of the physiological units of an organism to arrange themselves in the form proper to that organism. But we need not recognize in this tendency any such mystic entity as an Archæus or a vital
