INTRODUCTION 3 



in its own age, it has attained the final goal, at 

 a point which is, in reality, but a step in an endless 

 progress. 



When we watch the fate of one scientific hypothesis 

 after another as they pass across the stage from a 

 life of active usefulness into the historical museum 

 of intellectual curiosities, we learn that not even 

 the youngest generation of the sons of men is to 

 be deemed infallible. The theory by which know- 

 ledge is best advanced at any given time is not 

 necessarily more permanent than its predecessors. 

 The best theory is only the one which suits best the 

 existing modes of thought, and suggests most clearly 

 the particular direction in which advance may be 

 made. 



In natural science, as elsewhere, it is necessary to 

 distinguish between the passing and the permanent. 

 If the theories by which knowledge has been inter- 

 preted and extended are subject to flux and change, 

 knowledge of nature itself stands for no transitory 

 gain. The foundations are surely laid. Science has 

 continued to expand, and to show interconnections 

 and concordances between its parts which give con- 

 fidence in the stability of the whole. 



This steady growth in effective knowledge, and the 

 maintenance by all competent observers of a common 

 belief in fundamentals, are the principal features 

 which distinguish science from philosophy and meta- 

 physics. The attainment of general agreement is 

 indeed the crucial point at which a subject passes 

 from the realm of philosophic conjecture into the 

 territory of scientific " fact." 



All races at a certain stage of development treat 



