NATURAL SELECTION OF SPECIES. 161 



vention, and therefore examples of Nature's 

 method of selecting races are few ; but we 

 know that the size and quality of domestic 

 animals depend on the quality of their 

 food and conditions of life. Graziers, for 

 example, can tell by the appearance of cattle 

 or sheep the character of the pasture on 

 which they have been reared. The superior 

 quality of Lochleven trout, so well known to 

 the angler and the epicure, is due to the 

 character of the bed of the lake. Its fertile 

 mud is covered with highly nutritious aquatic 

 plants, on which molluscs and other forms 

 of life find abundance of food ; these, again, 

 provide generous nutrition for fishes, and 

 they respond both in number and quality. 

 Trout in streams or lakes on mountains com- 

 posed principally of granitic rocks also ex- 

 emplify the law, but in the opposite direc- 

 tion. Neither the soil nor the water in 

 such localities contains much nutrition for 

 plants or the lower forms of life ; food 

 is in consequence scanty and innutritious, 

 and the trout small in size and poor in 

 quality. 



It is, however, in the vegetable world that 

 Nature's law of selection can be most easily 

 studied. Among the Rothamstead agricul- 

 tural experiments one of the most interest- 



L 



