tions. The phenomena of immunity, especially to bacterial poisons that are so conspicuous in modern medicine, are adaptations of this type. It is still too early to state with any certainty the exact nature of the processes involved in such cases. That they are physico-chemical processes of great complexity seems to be clear. In this respect they ally themselves with the well-known equilibrium reactions in chemistry, and the form changes that certain crystals undergo in response to changes in temperature. Here, in the inorganic world, are relatively simple analogues, at least, of the physiological processes that are associated with adaptation in organisms. It is significant of the present attitude toward problems of adaptation, that suggestions for their solution are being thus eagerly sought among the facts of physics and chemistry.
VI
Scientific truth, then, is not concerned with final solutions. Nothing perhaps has been more conspicuously characteristic of it, in this discussion, than its incompleteness, than its plasticity, than its capacity for indefinite expansion, than its stimulating power. To my mind, this last is its crowning glory. We dwell in a world of hypotheses, and we estimate them according as they are more or less workable. To those hypotheses that approximate most closely to the demands of wide ranges of fact, we give the name of laws. It is obvious, however, that such laws nave varying degrees of certainty. Scientific truth is never absolutely certain, but there are always ways of determining what it may do.
For one who seeks a basis of criticism for a contribution to science, three obvious tests may be applied. (1) It may contribute new facts; (2) it may contribute a formulation of old facts; (3) it may contribute a new idea that, in the presence of facts, may lead to a new point of departure for explorations into the unknown.
If one were to apply these tests to what seem to me to be the two most significant developments in the philosophic thought of to-day, they might be said to fall, very roughly speaking, under the second and third categories. In the former might be placed the synthetic philosophy of Spencer, an avowedly scientific philosophy, whose essential problem was to formulate the known facts of science in term of principles of evolution. This stupendous project, remarkable alike for the powers of its author and the wide range of his interests, ended in a system of philosophy, into which just enough metaphysics succeeded in creeping to justify the criticism that, in spite of all good intentions, he had not been able completely to disentangle himself from the habits of thought to which his critics were happily accustomed.
In the third category may be placed that interesting application of
