236 A HISTORY OF FISHES 



are the naked and degenerate members of the family of Mailed 

 Cat-fishes {Cyclopium) ^ found only in the torrential creeks and 

 rivers of the Andes, most of them merely a succession of falls, 

 cascades, pot-holes and short "riffles"; and the Cyprinids, 

 Loaches, Suckers and Cat-fishes of the hills of Asia, India, 

 and the Malay Archipelago. 



Some idea of the difficulties encountered by fishes taking up 

 their abode in hill-streams may be gained if it is borne in mind 

 that huge volumes of water have to be carried away by a number 

 of relatively tiny streams after heavy falls of rain. In the region 

 of the Khasi Hills in India the average annual rainfall is no 

 less than 458 inches, and so great is the rush of water at times 

 that huge blocks of rock measuring four feet across have 

 been described as rolling along almost as easily as pebbles in 

 an ordinary stream, while the torrent of water is said to be 

 actually turbid with pebbles of some inches in size, suspended 

 in the water like mud. Describing the creeks and rivers of the 

 Andes, Mr. Johnson remarks that "the heavy rainfall, at times 

 amounting to four or five inches within a few hours, produces 

 flood of immense volume. These go charging down the 

 canyons with fearful fury, and at times it would appear that 

 nothing could withstand their sweeping energy." 



Thus, the strength of the current of water is the principal 

 factor influencing the evolution of fishes in torrential streams, 

 but food is also of primary importance, for, although quite 

 abundant, this consists largely of fine weeds and slime covering 

 the rocks and stones of the bed of the stream. No other 

 type of vegetation is able to avoid being swept away by the 

 force of the current, and although insect larvae are found in 

 fair numbers in certain regions and form an important article 

 of fish diet, the ultimate source of food is vegetable. Another 

 adverse factor, but one of less importance, is the extraordinary 

 clearness and shallowness of the water, which means that during 

 the day the inhabitants have to endure an intense light. On 

 the credit side of the account may be reckoned the rocky nature 

 of the bottom, with its large boulders and pot-holes, forming 

 ideal hiding-places for small fishes, and the abundance of air 

 dissolved in the water, due to its rapid and constant motion. 



The structural modifications resulting from these conditions 

 are of special interest, and involve such diverse parts of the body 

 as the skin, scales, mouth, fins, intestines, and air-bladder. 

 Nearly all are connected with the need for providing against 

 the danger of being swept away by the force of the current, 



