TENACITY OF LIFE. 187 



same physical conditions (Grayling, Salmo kucho). Some 

 marine fishes may be abruptly transferred from salt into 

 fresh water, like Sticklebacks, some Blennies, and Cottus, etc. ; 

 others survive the change when gradually effected, as many 

 migratory fishes ; whilst again, others cannot bear the least 

 alteration in the composition of the salt water (all pelagic 

 fishes). On the whole, instances of marine fishes voluntarily 

 entering brackish or fresh water are very numerous, whilst 

 freshwater fishes proper but rarely descend into salt water. 



Abstinence from food affects different fishes in a similarly 

 different degree. Marine fishes can endure hunger less than 

 freshwater fishes, at least in the temperate zones, no obser- 

 vations having been made in tliis respect on tropical fishes. 

 Goldfishes, Carps, Eels, are known to be able to subsist 

 without food for months, without showing a visible decrease 

 of bulk ; wliilst the Trigloids, Sparoids, and other marine 

 fishes, survive abstinence from food for a few days only. In 

 freshwater fishes the temperature of the water is of great 

 influence on their vital functions generally, and consequently 

 on their appetite, — many cease to feed altogether in the course 

 of the winter ; a few, like the Pike, are less inclined to feed 

 during the heat of the summer than when the temperature 

 is lowered. 



Captivity is easily borne by most fishes, and the appli- 

 ances introduced in our modern aquaria have rendered it 

 possible to keep in confinement, and even to induce to 

 propagate, fishes which formerly were considered to be in- 

 tolerant of captivity. 



Wounds affect fishes generally much less than liigher 

 Vertebrates. A Greenland Shark continues to feed whilst his 

 head is pierced by a harpoon or by the knife, as long as the 

 nervous centre is not touched ; a Sea-perch or a Pike (Fig. 97) 

 will survive the loss of a portion of its tail ; a Carp that 

 of half of its snout. However, some fishes are much more 



