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255 not been lacking) has been directed less to the accuracy of the formula as a description of fact than to the details of the process of reasoning by which Spencer deduces his formula from more fundamental principles.

This latter deductive treatment is much too complicated to make it possible for me to enter upon the discussion in a short essay. The last few chapters of First Principles consist of an attempt to show that the formula of evolution is a necessary consequence of the persistence of force: "To this an ultimate analysis brings us down, and on this a rational synthesis must build up." To those interested in this interpretation of evolution, these few chapters will supply an example of close and careful reasoning which well deserves study and attention. Whether this reasoning can be maintained in its entirety is a question on which I shall be excused from expressing a decided opinion where I am unable to make any attempt at an adequate exposition or discussion. While I am convinced that, in the main, the reasoning is sound, I do not now commit myself either to agreement or disagreement with the details of the argument. Whatever our opinion may be on this matter, there can be little doubt that the inductive verification is complete enough to warrant our assuming its truth provisionally and proceeding to the question of its practical value and validity.

It is from this standpoint that Spencer's principle is open to the most cogent criticisms, and at this point that a number of speculative problems arise. For the purposes of this discussion it will be convenient to compare Spencer's formula with the three physical principles which are generally admitted to be of universal applicability—the law of gravitation, the indestructibility of matter, and the conservation of energy. All these have one feature in common which makes them in that respect of