And just here we may glance at Spencer's definition of life as "the continual adjustment of inner relations to outer relations." Though emphasizing an important side of evolution, it is evidently incomplete. Evolution is not only the adjustment of inner relations to outer relations, it is also the adjustment of outer relations to inner relations as well as of inner relations among themselves; or it is a process of mutual adjustment of all the parts engaged in it. Our analysis, though crude and imperfect, may now be regarded as complete. Our scope will not allow of a more elaborate one. It is fitting, therefore, that we proceed to synthesis. The first matter which presents itself to us, in this connection, is the theory of Heredity and Adaptation mentioned above. The theory is not a new one, wholly outside Darwin's conception of evolution. The concept of Adaptation represents simply the generalization of all those special causes with which Darwin more particularly occupied himself, and is, in essence, only a proclamation of that universal subjection to natural law which Darwin himself plainly asserted. As such a. generalization it is, however, a useful one; it furnishes us with an expression, for the organic world, of that universal action and reaction through which opposing forces move towards stability by mutual adjustment. The law of Heredity, again, may be regarded as an organic expression of the more general principle according to which motion that, in the sense defined above, suffers only a minimum of interference, that is, motion which, by a certain equilibrium of mutual relations, is "approximately" or "fully" stable, tends to continue to take place in nearly the same directions, or nearly to repeat itself. It is thus apparent, also, that Heredity is closely related to the more special principle of Habit, or also of Use and Disuse, if only we remember that, whatever the metaphysical truths of Freedom or Determination, the psychical is always accompanied by what may be called equivalents of the physical under natural law. The special laws of Heredity are still enveloped in mystery; I refer, not to that mystery which may be regarded as surrounding all ultimate facts, if we choose to conceive them as expressing or concealing something further unknowable, but to the scientific mystery of ignorance, which time may dissolve. Biologists disagree on this question, the ultimate decision of which must be left to them. Still some general criticism on the results of research in this direction may be allowable from a philosophic standpoint.