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 comes to our perception consists chiefly of the abortive or unsuccessful attempts, when birds are checked in their course, and being unable to proceed present themselves to our sight and hearing. Now for obvious reasons birds could not well fly at very great heights in very thin air, as experiments with pigeons released from balloons have shown, and the condor soaring far above the tops of the Andes is a myth. The few trustworthy instances in which birds have been observed through a telescope passing across the face of 'the moon have naturally yielded but vague calculations as to distance and height. W. E. D. Scott (Bull. Nuttall Orn. Club, vi. 97–100), computed heights varying from 1 to 2 m. F. M. Chapman’s observations (Auk, 1888, pp. 37–39) resulted in a height of from 1500 to 15,000 ft.; average, say, 1 m. If the sky is clouded and the birds fly above the clouds the migration proceeds beyond our ken, and if for some reason or other they are below the clouds the phenomenon becomes to us very noticeable. It is well known “that on clear and bright nights birds are rarely heard passing overhead, while on nights that are overcast, misty and dark, especially if slight rain be falling, flocks may often be heard almost continuously.” It is in such weather, continues Newton, that birds while migrating are most vociferous, doubtless with the result that thereby the company of fellow-travellers is kept together.

There yet remain a few words to be said on what may be termed Exceptional Migration, that is when from some cause or other the ordinary practice is broken through. The erratic movements of the various species of crossbill (Loxia) and some allied forms afford perhaps the best-known examples. In England no one can say in what part of the country or at what season of the year he may not fall in with a company of the common crossbill (L. curvirostra), and the like may be said of many other lands. The food of these birds consists mainly of the seeds of conifers, and as its supply in any one locality is intermittent or precarious, we may not unreasonably guess that they shift from place to place in its quest, and may thus find an easy way of accounting for their uncertain appearance. The great band of nutcrackers (Nucifraga caryocatactes) which in the autumn of 1844 pervaded western and central Europe (Bull. Acad. Bruxelles, xi. 298), may also have been actuated by the same motive, but we can hardly explain the roaming of all other birds so plausibly. The inroads of the waxwing (Ampelis garrulus) have been the subject of interest for more than 500 years, and by persons prone to superstitious auguries were regarded as the forerunners of dire calamity. Sometimes years have passed without the bird being seen in central, western or southern Europe, and then perhaps for two or three seasons in succession vast flocks have suddenly appeared. Later observation has shown that this species is as inconstant in the choice of its summer as of its winter-quarters. One of the most extraordinary events known to ornithologists is the irruption into Europe in 1863 of Pallas’s sand-grouse (Syrrhaptes paradoxus). Of this bird, hitherto known only as an inhabitant of the Tatar steppes, a single specimen was obtained at Sarepta on the Volga in the winter of 1848. In May 1859 a pair is said to have been killed in the government of Vilna, on the western borders of the Russian empire, and a few weeks later five examples were procured, and a few others seen, in western Europe—one in Jutland, one in Holland, two in England and one in Wales. In 1860 another was obtained at Sarepta; but in May and June 1863 a horde computed to consist of at least 700 individuals overran Europe—reaching Sweden, Norway, the Faeroes and Ireland in the north-west, and in the south extending to Sicily and almost to the frontiers of Spain. On the sandhills of Jutland and Holland some of these birds bred, but they were all killed off. A much greater visitation took place in 1888, which met with the same fate. The number of birds was quite incalculable, the wave extending from Norway to southern Spain.

In comparison with the periodic annual migrations of so very many birds, those of other creatures are scarce and insignificant, excepting fishes.

Mammals.—Few trustworthy observations have been recorded. The most regular and least limited migrations seem to be those of the eared seals. The Walrus also goes each year to the north in the summer, further south in the winter. Delphinapterus leucas, one of the Cetacea, ascends the Amoor regularly on the breaking of the ice, a distance of 400 m. up the stream, Some bats are supposed to migrate. The American bison used to roam north and south, according to the season, in search of pasture; and similar periodic wanderings have often been recorded of various kinds of game on the South African veldt. They are all obviously a mere matter of commissariat and have little to do with the breeding, except in the case of seals.

In one way the lemming’s “migrations” are instructive. They are quite sporadic. When, owing to combination of some favourable circumstances they suddenly increase, enormous numbers forsake the highlands for the lowlands of Norway; not in a methodical way, but quite lawlessly; that means to say they radiate from their centres of dispersal. At any given spot, however, they seem to keep to the same direction, and no obstacles seem to divert their course. Those which arrive at the much indented coast are known even to rush into the sea, where of course they get drowned. There is no sense in this. The overcrowded condition of their home impels them to leave, and this impulse continues blindly. They do not attempt to settle anywhere between their home and the sea. A year or two after the irruption not a lemming is there to be found, and where during their stampede they come across suitable districts, they find these already occupied by resident lemmings.

Such and similar irruptions have no doubt taken place often during the world’s history; and yet such sporadic stampedes into a foreign country hardly ever lead to its regular settlement, especially when such a country possesses already a kindred fauna of its own.

Fishes.—Many fishes make periodic migrations for breeding purposes, which by their numbers and the distances travelled much resemble those of birds, but very little is known about these fishes. Take the incredible masses of herrings and their kindred; the collecting of the cod and its allies on their breeding-ground. According to D. S. Jordan (A Guide to the Study of Fishes, New York, 1905) some kinds are known mainly in the waters they make their breeding-homes, as in Cuba, southern California, Hawaii or Japan, the individuals being scattered at other times through the wide seas. The tunny, which has a world-wide distribution, arrives off the south coast of Portugal in the month of May; enormous numbers pass through the Straits of Gibraltar and support great fishing industries in the Mediterranean. In the month of August they return to the ocean (Apesca do Atum no Algarve em 1898, por D. Carlos de Braganza, Lisboa, 1899; with many maps).

Many fresh-water fishes, as trout and suckers (quoting Jordan) forsake the large streams in the spring, ascending the small brooks where their young can be reared in greater safety. Still others, known as anadromous fishes, feed and. mature in the sea, but ascend the rivers as the impulse of reproduction grows strong. Among such fishes are the salmon, shad, alewife, sturgeon and striped bass in American waters; Clupea alosa, the Allis shad, and C. finta, the Twait shad, Alepocephalus rostratus, the “maifisch” of the Rhine, in Europe. “The most remarkable case of the anadromous instinct is found in the king-salmon or quinnat (Onchorhynchus tschawytscha), of the Pacific coast. This great fish spawns in November, at the age of four years and an average weight of twenty-two pounds. In the Columbia river it begins running with the spring freshets in March and April. It spends the whole summer, Without feeding, in the ascent of the river. By autumn the individuals have reached the mountain streams of Idaho, greatly changed in appearance, discoloured, worn and distorted, On reaching the spawning-beds, which may be 1000 m. from the sea in the Columbia, over 2000 m. in the Yukon, the female deposits her eggs in the gravel of some shallow brook. The male covers them and scrapes the gravel over them. Then both male and female drift, tail foremost, helplessly down the stream; none, so far as certainly is known, ever survive the reproduction act. The same habits are found in the five other species of salmon in the Pacific. The salmon of the Atlantic has a similar habit, but the distance travelled is everywhere much less, and most of the hook-jawed males drop down to the sea and recover, to repeat the act of reproduction.”,

Few fishes are katadromous, i.e. their usual habitat is in rivers and lakes, but they descend into the sea for breeding purposes. The common eel is the classical example.

Insects.—D. Sharp makes the following remarks (Cambridge Nat. Hist. vi.): “Odonata are among the few kinds of insects that are known to form swarms and migrate. Swarms of this kind have been frequently observed in Europe and in North America; they usually consist of a species of the genus Libellula, but species of various other genera also swarm, and sometimes a swarm may consist of more than one species.

“Locust swarms do not visit the districts that are subject to their invasions every year, but as a rule only after intervals of a considerable number of years The irregularity seems to depend upon three facts, viz. that the increase of locusts is kept in check by parasitic insects; that the eggs may remain more than one year in the ground and yet hatch out when a favourable season occurs, and that the migratory instinct is only effective when great numbers of superfluous individuals are produced It is well established that locusts of the migratory species exist in countries without giving rise to swarms or causing any serious injuries When migration of locusts does occur it is attended by remarkable manifestations of instinct. Although several generations may elapse without a migration, it is believed that the locusts when they migrate do so in the direction taken by predecessors. They are said to take trial flights to ascertain the direction of a favourable wind, and that they alight and wait for a change. The most obscure point is their disappearance from a spot they have invaded. A swarm will alight on a locality, deposit there a number of eggs, and then move on. But after a lapse of a season or two there will be few or none of the species present in the spot invaded. In other cases they again migrate after growth to the land of their ancestors. It has been ascertained by the United States Entomological Commission that such return swarms do occur.”

See J. A. Palmén, Om Foglarnes flyttningsvägar (Helsingfors, 1874). The same in German: Über die Zugstrassen der Vögel (Leipzig, 1896). In this and the work of von Middendorff, already