THE LATEST STAGE 281 



which suggested them, are essentially provisional. 

 They are but working hypotheses adapted to the needs 

 of the passing age. What part of the conceptual 

 model seemed better adapted to stand as a permanent 

 representation of nature than the immutability of 

 the chemical elements, what surer than the Newtonian 

 dynamics ? Yet, under the pressure of radio-active 

 phenomena, we have been forced to repudiate the one 

 as a universal law, and to modify the other to explain 

 the motion of bodies which approach too nearly the 

 velocity of light. 



Again and again in the course of the preceding 

 chapters we have traced how a theory, put forward 

 at a definite stage in the development of a subject, 

 interprets adequately and successfully existing know- 

 ledge and points the way for future research. Yet, 

 a few pages later, we are driven to recount its failure 

 under the test of ever-advancing thought, and its 

 supersession by some other hypothesis more adapted 

 to the needs of the new time, and more fruitful in 

 suggestions for further work. 



Moreover, we have had continually to acknowledge 

 that the later theory would have been less useful at 

 the previous stage than the outworn idea it replaced. 

 We come to see that the test of a successful theory is 

 not its concordance with absolute truth with which 

 indeed we can never compare it but the humbler and 

 more practically useful role of giving us a means of 

 co-ordinating conveniently and succinctly our existing 

 unconnected pieces of knowledge, and of enabling us 

 to frame experiments and enquiries calculated to open 

 up the greatest extent of hitherto unsurveyed territory 

 in the kingdom of nature. The test of a theory, then, 



