EVIDENCE FROM EXPERIMENT 169 



of hypotheses and theories, which have served their 

 purpose and have been worn out and discarded. 

 How are we to distinguish between those theories 

 which will be permanent and those which are 

 doomed to decay and oblivion? A theory is only 

 an attempt to explain and interpret a body of facts 

 and, for a time, this may be equally well done by a 

 true theory and a false one; both may seem equally 

 plausible and promising. It is the advance of dis- 

 covery, bringing to light many new facts, which 

 were neither known nor anticipated when the rival 

 theories were first propounded, that puts the theories 

 to the severest test. From this point of view, the 

 doctrine of evolution has stood the test remarkably 

 well. Darwin's book put new life into all depart- 

 ments of biology; every one, friends and foes alike, 

 was eager to find proofs which should establish or 

 overthrow this new and innovating belief, so that 

 all lines of inquiry were pushed forward with re- 

 newed activity and enthusiasm. This zealous labour 

 has resulted in the accumulation of a mass of data 

 such as Darwin never dreamed of. The existence 

 of the marvellous treasures which have been disin- 

 terred from the rocks of western North America 

 was hardly suspected when the " Origin of Species " 

 appeared, yet it is there that were found some of the 

 most complete genealogical series yet made known. 

 Though Darwin himself was one of the pioneers in 

 the discovery of South American fossil mammals, 

 it was not until after his death that the immense 



