DARWIN'S VIEWS OF METHOD. 43 



system or in the sun, and acted with a force 

 varying inversely as the square of the dis- 

 tance; and that they could not be what they 

 are if the force were located anywhere except 

 at the centre. In this instance Newton was 

 able to exclude mathematically every theory 

 except the true one; and the demonstration 

 was made complete by the positive proof that 

 the facts accorded with the one hypothesis and 

 were at variance with every other. 



Darwin used the principle of exclusion in 

 one of his early scientific efforts. There were 

 two hypotheses to account for the existence of 

 the great terraces called benches, or parallel 

 roads, of Glen Roy, in Scotland. It was evi- 

 dent enough that they had been formed by the 

 action of water; and they must either have 

 been formed by the sea, in which case the 

 uppermost, for example, must have been after- 

 wards elevated more than one thousand feet; 

 or they must have been the ancient shores of 

 a lake formed by the blocking up of the 

 open side of the now empty lake bed. Darwin 

 studied the region, and concluded that the 

 benches were of marine origin. He could not 

 conceive them to be due to barriers of rock or 

 detritus. He was then fresh from his studies 

 on the geology of South America. On the 



