Then there are migratory fishes which live part of the year in the rivers. Such are the shad and the river-herrings or alewives, which leave the sea in the spring and ascend to the river heads to spawn, and the salmon, which does likewise, to spawn in the brooklets in November and December. Still more remarkable is the eel, which breeds in the sea, where the male eels always remain, while the young females, when as large as darning needles, ascend in the spring to inland lakes and streams, there to remain, until, after three or four years, they are grown to maturity, when they descend to salt water, to reproduce their kind and die. There are also migratory fish in fresh water, like the white fish, the salmon, trout, and the siscowet, which live in the abyssal depths of the great lakes and swim up into the shallows and creeks in winter to spawn their eggs, and the brook trout and dace, which for a similar purpose ascend from the pools and quiet meadow stretches to the pebble-paved ripplets near the spring sources of the brooks in which they live. Having, in a general way, classified fish according to their habits, we are in a position to consider the manner in which man has succeeded in exterminating them. As a general rule, fish deposit their eggs in shallow water, and the time of egg-laying is very closely dependent upon the temperature of the water. The eggs of a fish are, as every one knows, enclosed in two sacs, or ovaries, which are situated close to the walls of the abdominal cavity, and separated from the water by thin walls of skin and flesh, rarely, even in the largest fish, more than a sixteenth of an inch in thickness. Experiment has shown that the temperature of the blood in the abdomen of a fish deviates very little from that of the water in which it is floating. Experiment has also shown that as soon as the water has reached a certain degree of warmth, variable with each kind of fish, the eggs are sure to be laid within a very few hours. This being the case, it usually happens that great schools of fish always congregate together at one time upon the spawning grounds. Since the spawning grounds of many kinds of fish are in shallow water, and the fish are at that time most easily caught, it happens that many of the most extensive fisheries are carried on in the spawning season. The delicious little smelts which our neighbors in Maine and New Brunswick send us by the hundred car-loads each winter, packed in little boxes of snow, are always full of eggs—so are the lake white fish, when they are caught, and the shad, and the Potomac herrings, and the cod, and the mullet, and the herring, and in early spring the mackerel.