174 VIGNETTES FROM NATURE. 



and peculiar way. In the rivers of the low- 

 lands, trout are exposed to the attacks of pike 

 and other savage predaceous fish ; here, they 

 need only fear the herons and the angler. 

 In the lowlands, they hide among weed or 

 under banks ; here, they are exposed in full 

 sunshine against a light weedless gravelly 

 bottom. In the lowlands, they feed largely 

 upon land worms and other straggling prey ; 

 here, they subsist almost entirely upon flies 

 and other winged insects. Accordingly, the 

 qualities which ensure success in the one 

 habitat are quite different from those which 

 ensure success in the other ; and as the suc- 

 cessful alone survive and propagate their like, 

 it is not surprising that ever since the end of 

 the last ice age — probably no more than 

 some 70,000 years since — as many as eight 

 or ten separate species of mountain-tarn trout 

 should have been evolved in the British Isles 

 alone. 



At the same time, it must be borne in 

 mind that while the isolated colonies in these 



