44 THE METHOD OF DARWIN. 



coasts of that continent he had grown familiar 

 with the immense scale on which elevations 

 take place, and had many opportunities to 

 study marine sand and gravel formations that 

 are now hundreds of feet above sea level. It 

 was therefore as easy for him to conceive a 

 marine origin of the terraces of Glen Roy as 

 it was difficult for him to believe that they 

 were the old shores of an elevated lake that 

 had been blocked in. He adopted the method 

 of exclusion, proved the one hypothesis prob- 

 able and the other improbable. 



But this instance of the principle of exclu- 

 sion illustrates well the danger to which the 

 investigator is exposed in its use. In geology, 

 zoology, etc., in what Darwin calls the mixed 

 sciences, one can rarely know whether all 

 the possible hypotheses are known. Newton 

 could make a rigid demonstration, not simply 

 because he could treat his problem mathe- 

 matically, but because he was able to treat 

 all the possible hypotheses. When the gla- 

 cial theory was introduced into the geological 

 world the ancient terraces high above sea 

 level in the colder temperate zone were ac- 

 counted for. Then it was plain that the ter- 

 races of Glen Roy had been the shores, not 

 of the sea, nor of a lake dammed up by rock 



