Fishes differ in an extraordinary degree with regard to tenacity of life. Some will bear suspension of respiration--caused by removal from water, or by exposure to cold or heat--for a long time, whilst others succumb at once. Nearly all marine fishes are very sensitive to changes in the temperature of the water, and will not bear transportation from one climate to another. This seems to be much less the case with some freshwater fishes of the temperate zones: the Carp may survive after being frozen in a solid block of ice, and will thrive in the southern parts of the temperate zone. On the other hand, some freshwater fishes are so sensitive to a change in the water that they perish when transplanted from their native river into another apparently offering the same physical conditions (Grayling, _Salmo hucho_). 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 observations having been made in this 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; whilst 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 appliances 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 intolerant of captivity. Wounds affect fishes generally much less than higher 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 sensitive, and perish even from the superficial abrasion caused by the meshes of the net during capture (_Mullsn_.)