172 VIGNETTES FROM NATURE, 



— which latter metaphor he does not deign 

 to explain for us in full. For my own part, 

 I do not incline thus clumsily to solve the 

 problem with a dens ex machind ; one cannot 

 fairly consider it a digmis vindice nodus. It 

 seems to me more likely that when the fish 

 first came here the little stream still flowed 

 in a moderately continuous basin, worn for it 

 by the glacier, down to the level of the 

 river, which then ran in a far higher channel 

 than at present. Up this gentle incline the 

 trout which were slowly spreading through 

 the unoccupied fresh waters of Britain, after 

 the thawing of the great ice-sheet, must have 

 made their way into Llyn Gwernant. 



But when they had once got there, the 

 brook and the river went on carving their 

 basins through the rock and the glacial soil, 

 till at last they reached their present levels, 

 the three highest falls on the brook being 

 just those where it meets the newer valley of 

 the main stream by Dolserau Mill. So after 

 a while no more trout could reinforce the 



