The crustaceous tribes have also furnished much matter of observation. The Prawn and Crab, for instance, exercises the virtue of conjugal fidelity to the highest degree; for the male takes hold of his mate, and never lets her go; he swims with her, crawls about with her, and if she is forcibly taken away from him, he seizes hold of her again. The metamorphoses to which various crustaceous tribes are subject have also been studied with much attention.[16] Much as the nature and habits of fish have been studied of late years, the economy of some is to this day involved in obscurity. The Herring is one of these fishes. The Swedish Herring Fisheries were, at one time, the largest in Europe, but at present, during the temporary disappearance of the fish, they have dwindled away. The causes which influence the movements of the Herring--one of the most capricious of fish--are a puzzle which naturalists have as yet failed to solve. They are not migratory, as was at one time believed--that is, they seldom wander far from the place where they were bred; but they are influenced by certain hidden and unexplained causes at one time to remain for years in the deep sea, and at another to come close in to land in enormous numbers. During the first half of the sixteenth century, Herrings entirely deserted the Swedish coasts. In 1556 they reappeared, and remained for thirty-one years in the shallow waters. Throughout this period they were taken in incalculable numbers; "thousands of ships came annually from Denmark, Germany, Friesland, Holland, England, and France, to purchase the fish, of which sufficient were always found for them to carry away to their own or other countries.... From the small town of Marstrand alone some two million four hundred thousand bushels were yearly exported." In 1587 the Herrings disappeared, and remained absent for seventy-three years, till 1660. In 1727 they returned, and again in 1747, remaining till 1808, and during this last period the fisheries were prosecuted with extraordinary zeal, industry, and success. The Government gave every encouragement to settlers, and it was computed that during some years as many as fifty thousand strangers took part in them. In 1808 the Herrings once more disappeared, and have never returned since. The cause must still be considered as quite unknown; but we may fairly assume, according to historical precedents, that after a certain period of absence, the Herrings will again return.[17] Aristotle, in his "History of Animals," makes some extremely curious observations on Fish and Cetaceous Animals, as might be expected from the variety of these animals in the Grecian seas. In Spratt and Forbes's "Travels in Syria" the account of the habits and structure of the Cuttle-fish in Aristotle's work is ranked amongst the most admirable natural history essays ever written. It is, moreover, remarkable for its anticipation.