Some of the marine turtles are good instances of true migrants. Thus the loggerhead (Caretta caretta), a carnivo rous species of tropical and intertropical seas, visits many sandy shores to deposit the eggs on the beach. The newly hatched young make persistently for the sea, moved, as G. H. Parker has shown, by a constitutional obligation to go down a slope and to walk in the direction of the most open low horizon. The vegetarian green turtle (Chelone midas), which deposits its eggs on sandy beaches in the West Indies, has a migrational range not known to exceed so miles. It spends ten months of the year in the relatively shallow coastal waters, for though sometimes found in the open ocean, its normal haunt is bound to be not very far from the seaweed growing region. Some of the sea-snakes are true migrants, coming periodically to shore to give birth to their young among the rocks. Amphibians.—A familiar sight in spring in some places is the march of toads from a considerable distance to a particular pond or marsh where they breed. When, some months later, the tad poles have become small toads, there is a journey in the opposite direction, the parents having returned inland some time pre viously. The same is true of the crested newt in Britain, but the distances covered are not so great. Similarly in the common grass frog there is a summer movement from the water to the fields. The return to the water, or to the vicinity of the water, is autumnal, not vernal, as in toads, and it is not so well-defined. Fishes.—The term migration has been mistakenly applied to many mass-movements of fishes, e.g., herring and mackerel, which have no connection with a return to a particular spawning-ground or type of spawning-ground. These non-migrational movements
are largely explicable in terms of changes in the distribution of the planktonic and other organisms on which the fishes feed, or what may come to the same thing, in terms of changes in tempera ture, salinity, oxygenation, carbon dioxide tension, and so forth. No mass-movement of fishes should be ranked as migrational unless directly concerned with approaching or leaving a spawning area.
True migration is familiarly illustrated in the salmon (Salmo solar). The eggs are liberated, often in midwinter, on suitable gravelly stretches of the river-bed. There are successive stages of alevins, fry, parr and smolts. The last, when over two years old, pass down the rivers to the sea, usually in early summer. A vigor ous nutritive life is spent in the sea, where the food consists largely of herrings and mackerel; the salmon may remain there for several years. Adolescent salmon, which have not quite put on the adult characters are called grilse, and are normally three or three and a half years old, having descended to the sea as smolts the previous year. These may ascend the rivers and may spawn as grilse ; but the grilse stage is often passed through in the sea, so that the maiden fish entering the fresh water for the first time are often "salmon." The adult salmon eat very little, if at all, in fresh water ; they return to the sea, if they can, after spawn ing. In some cases it has been proved by marking that salmon return from the sea to their own particular native river (see