10 FLY-FISHING AND FLY-MAKING. 



is the adaptability. Indeed, these premises are truisms, 

 and would not need repetition did not we so habitually 

 forget that design is the key to all the natural phenomena 

 around us. Admitting this, it follows that the structure 

 of the eye of a fish and its position are admirably adapted 

 for seeing in a denser medium than air with great precision 

 and certainty. Observation ratifies this conclusion in its 

 entirety. The vitreous and crystalline humors of the 

 organ are not different from those of other animals. The 

 muscles moving the body of the eye are large and permit 

 great freedom, and the power of contraction and expan- 

 sion of the irides is also distinct and pronounced. In air, 

 this latter feature is, indeed, very noticeable in the case 

 of some fishes, and the angler has only to accurately 

 measure the breadth of the pupil of such fish in shaded 

 water, and compare this with the measurement of the 

 same after the fish has lain in full sunlight a few mo- 

 ments, to be at once forcibly impressed by the fact. The 

 position of the orbit also needs a word of comment. In 

 fish which readily and constantly rise and descend, to 

 feed, it is placed so as to command a large area around ; 

 in fact, as large as in the nature of the creature is pos- 

 sible. In such as chiefly rise to their prey, as the pikes, 

 it is placed quite near the top of the head. In the barbel 

 (Cyprinus barbus) and gudgeon (Golio fluviatilis) the 

 converse obtains. In the trout, however, the orbit is 

 more central, and the broad visual grasp of this fish, 

 therefore, is one of the reasons why it, in due time, be- 

 comes so wary and so capable of protecting itself against 

 its arch enemy man. A pike cannot possibly see its 

 ventral fin ; a trout undoubtedly could, if it felt so dis- 



