36 PROCEEDINGS MANCHESTER INSTITUTE 



gently controlled. The problem is intricate, and its factors, in 

 the present state of knowledge, obscure ; but the persistent and 

 extended application of scientific methods of investigation can- 

 not fail to result in its ultimate solution. A simple illustration 

 will afford some idea of the nature of the problem and of the possi- 

 bilities as well as of the difficulties involved. 



A certain brook in a town near Manchester v/as long celebrat- 

 ed for its trout. Even to this day a considerable number of in- 

 dividuals of good size are found there, notwithstanding the al- 

 most incessant fishing for them. A brook weed grows abund- 

 antly in the rapids of this stream, clinging to the stones at the 

 bottom. Snails in vast numbers feed upon this particular 

 weed. The reproductive capacity of snails is great, and this 

 brook weed is evidentl}^ a favorite food plant. It is known that 

 the larvs of horseflies (Tabanidse) feed upon these snails, atid, 

 as might be expected, horsefly larvae are abundant here. In 

 this particular brook the singular little fishes known as " dart- 

 ers " (in this case some species or variety of Etheostoma as yet 

 unidentified) are found in surprising numbers, lying half hidden 

 among the stones. These feed upon the horsefly larvse, and 

 are themselves noted among anglers as a special bait for trout.- 



Here we have an apparently simple chain of cause and effect, 

 from the brook weed to the trout. In most cases the inter-re- 

 lations are more complicated ; but even here the problem is not 

 so simple as at first appears. If we jump at the conclusion that 

 a stream which abounds in brook weed is therefore suitable for 

 trout, we may fall into the error of that well-meaning person 

 who, observing that civilized man uses the toothbrush, hastily 

 inferred that if savages were equipped with toothbrushes, they, 

 too, would be civilized. 



In this seemingly simple chain we must know what are the 

 conditions favorable to the growth of the brook weed, and whj- 

 it is abundant in one stream but wanting in another. Of the 

 snails we must know what other plants supply a portion of their 

 food, and at what seasons, as well as what enemies other than 

 the horsefly larvae tend to check their increase ; and we must 

 learn as much or more of the horseflies themselves before we 



