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A most curious and interesting set of phenomena are those which often occur when a group of fishes, once marine, take by practice to inhabiting freshwater rivers; or, vice-versa, when a freshwater kind, moved by an aspiration for more expansive surroundings, takes up its residence in the sea as a naturalised marine. Whenever such a change of address happens, it usually follows that the young fry cannot stand the conditions of the new home to which their ancestors were unaccustomed–we all know the ingrained conservatism of children–and so the parents are obliged once a year to undertake a pilgrimage to their original dwelling-place for the breeding season.

Extreme cases of terrestrial animals, once aquatic in habits, throw a flood of lurid light (as the newspapers say) upon the reason why this should be so. For example, frogs and toads develop from tadpoles, which in all essentials are true gill-breathing fish. It is, therefore, obvious that they cannot lay their eggs on dry land, where the tadpoles would be unable to find anything to breathe; so that even the driest and most tree-haunting toads must needs repair to the water once a year to deposit their spawn in its native surroundings. Once more, crabs pass their earlier larval stages as free-swimming crustaceans, somewhat shrimp-like in appearance, and as agile as fleas: it is only by gradual metamorphosis that they acquire their legs and claws and heavy pedestrian habits. Now there are certain kinds of crab, like the West Indian land-crabs (those dainty morsels whose image every epicure who has visited the Antilles still enshrines with regret in a warm corner of his heart), which have taken in adult life to walking bodily on shore, and visiting the summits of the highest mountains, like the fish of Deucalion’s deluge in Horace. But once a year, as the land-crabs bask in the sun on St. Catherine’s Peak or the Fern Walk, a strange instinctive longing comes over them automatically to return for a while to their native element; and, obedient to that inner monitor of their race, down they march in thousands, velut agmine facto, to lay their eggs at their leisure in Port Royal harbour. On the way, the negroes catch them, all full of rich coral, waiting to be spawned; and Chloe or Dinah, serves them up hot, with breadcrumbs, in their own red shells, neatly nestling between the folds of a nice white napkin. The rest run away, and deposit their eggs in the sea, where the young hatch out, and pass their larval stage once more as free and active little swimming crustaceans.

Well, crabs, I need hardly explain in this age of enlightenment, are not fish; but their actions help to throw a side-light on the migratory instinct in salmon, eels, and so many other true fish which have changed with time their aboriginal habits. The salmon himself, for instance, is by descent a trout, and in the parr stage he is even now almost indistinguishable from many kinds of river-trout that never migrate seaward at all. But at some remote period, the ancestors of the true salmon took to going down to the great deep in search of food, and being large and active fish, found much more to eat in the salt water than ever they had discovered in their native streams. So they settled permanently in their new home, as far as their own lives went at least; though they found the tender young could not stand the brine that did no harm to the tougher constitutions of the elders. No doubt the change was made gradually, a bit at a time, through the brackish water, the species getting further and further seaward down bays and estuaries with successive generations, but always returning to spawn in its native river, as all well-behaved salmon do to the present moment. At last, the habit hardened into an organic instinct, and nowadays the young salmon hatch out like their fathers as parr in fresh water, then go to the sea in the grilse stage and grow enormously, and finally return as full-grown salmon to spawn and breed in their particular birthplace.