98 THE POPULAR SCIENCE MONTHLY 



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 asso- 

 ciated with adaptation in organisms. It is significant of the present 

 attitude toward problems of adaptation, that suggestions for their solu- 

 tion are being thus eagerly sought among the facts of physics and chem- 

 istry. 



VI 



Scientific truth, then, is not concerned with final solutions. Nothing 

 perhaps has been more conspicuously characteristic of it, in this dis- 

 cussion, 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 evolu- 

 tion. This stupendous project, remarkable alike for the powers of its 

 author and the wide range of his interests, ended in a system of philoso- 

 phy, 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 



