3 82 INFLUENCES OF CULTIVATION 



fishes. It should be studied along with the succeeding map, 

 to reveal the full measure of man's interference in our fresh 

 waters. 



POLLUTION OF RIVERS 



Perhaps a more comprehensive destruction of aquatic 

 life has followed upon the discharge into rivers of harmful 

 by-products of manufacture. It is difficult to gauge the 

 cumulative effects of the continuous flow of dilute solutions 

 of poisonous by-products, and the subject has aroused much 

 controversy ; but many a river is made unsightly by their 

 presence, and there can be little doubt that in such cases 

 the organic life of the river is adversely affected. There 

 can be no doubt that where deleterious by-products are 

 allowed to enter rivers in quantity, even if intermittently, 

 there results a destruction of fishes, and, a point as important 

 though often forgotten, a destruction of the small creatures 

 which are the food of fishes. 



The pollution of rivers in a degree sufficient to affect 

 the fauna is to a large extent a development of modern 

 civilization, for although pollution follows upon the growth 

 of industry and upon the aggregation of the people in social 

 centres, yet its serious influence is a matter of quantity as well 

 as of quality. As a rule it is due to the discharge from mills, 

 mines, or sewage works of substances actually or potentially 

 harmful, into the water of streams or rivers. Such dis- 

 charges tell upon animal life in more ways than one. 



Solid substances cast into rivers, even where, as in the 

 case of sawdust, they seem to do no actual harm to adult 

 fishes, yet cover the bed of the stream with layers of debris 

 which ruin potential spawning beds and in others destroy 

 fish eggs by preventing free access of oxygen. The de- 

 position of solid matter in streams is illegal in Britain, yet 

 here similar effects follow upon the discharge of washings 

 from mineral works which contain fine dust in suspension. 

 Thus the beds of streams in the neighbourhood of many coal 

 mines are buried in layers of coal dust which settles every- 

 where, stifling vegetation and its animal dependents, upon 

 which in turn the larger inhabitants of the streams subsist, 

 destroying spawning beds, and actually choking fishes which 

 venture within the noxious area. So also it is with the 



