A MOUNTAIN TARN, 175 



little pools have been slowly altering in one 

 direction under the influence of changed con- 

 ditions and of a more specialised natural 

 selection, the trout of the lowland rivers have 

 also doubtless been altering in another 

 direction, under the influence of stronger and 

 fiercer competition. The divergence has 

 been double-sided. We must not take the 

 existing lowland trout for a true representative 

 of the common ancestor, and then measure 

 the deviations of this. Llyn Gwernant species 

 by that fallacious standard. If ever a young 

 Llyn Gwernant troutlet, in his desire to see 

 the world, leaps the cascades and ventures 

 down into the river, we may be sure he is 

 snapped up bodily by the first pike that meets 

 him ; and that is why this rare species has 

 never spread elsewhere. It is only suited to 

 its own habitat ; while the common speckled 

 trout of our rivers are adapted to avoid the 

 various greater dangers of their wider world. 

 If we were to compare together all the special 

 mountain trout of the various Welsh and 



