The evolution of the study and knowledge of migration is an interesting subject, dealt with more or less completely by several writers. In a manual it is impossible to treat it fully. That the Greek poets—Homer and Anacreon for instance, and the writers of Jeremiah and Job, knew something about the regular movements of birds is evident, nor is it surprising that in lands like Greece, Egypt and Palestine the passage of birds should be noted and directly connected in the popular mind with the seasonal changes.

In a measure similar observations and conclusions may be traced in the history or traditions of most peoples, but in a northern detached area, such as the British Islands, there is a marked tendency to overlook passage and note only arrival and departure, mostly of summer birds. Early observers noticed the swallow and cuckoo when they had actually come, and missed them when they had gone, but they failed to grasp whence they came or whither they went. Interchange of ideas with inhabitants of other lands was limited, and few early travellers were philosophers, at any rate so far as migration was concerned. In Germany, however, the Emperor Frederic II. realised in the thirteenth century many truths concerning migration (27), but in Britain uncertainty or myth held sway until the end of the eighteenth century. Herr Herman, reviewing the variation in thought, says—"But as in other fields, this period is followed by a time of decadence, a natural consequence of departing from immediate experience."

British, and many Continental observers too, saw when birds had come and in autumn that they had gone. Early swallows and martins were always met with near water, and were watched dropping to roost in the reed beds, as they always do in autumn before departure. Next morning none was visible. Certainly then they had vanished to hibernate in the water. The discovery of masses of torpid swallows, dead or dying, by no means an unknown thing when birds are overtaken by sudden falls in temperature in autumn or by a severe setback in the spring, was to these puzzled men confirmation of their theory of hibernation. Other details of the many stories of swallow hibernation are due to exaggeration or to misconception. In the second half of the eighteenth century a fierce discussion waged for and against hibernation, and many, including Geoffroy St Hilaire and Montagu, sat on the fence, admitting that it might be possible with some species and probably was with swallows. Later some Americans produced "evidence" in favour of avian hibernation, and even Mr Charles Dixon, in his earlier book at any rate, did not think it impossible (25). The only argument in favour of hibernation is that it is a habit resorted to by other vertebrates to escape the consequence of exposure to severe temperatures. The arguments against it are that not a single instance of avian hibernation will stand the light of reason and investigation, and that birds are provided with the means of escaping from the cold zone and certainly use these means. There are flightless birds, but they all live in climates in which they can exist at all seasons. As Seebohm puts it—"The hibernation of birds is a theory, the evidence in support of which has completely broken down. The migration of birds is a fact, as completely authenticated as the fact of their existence."

Dr Derham's "Physico-Theology" appeared in 1737 (24), and contained some sound reasoning about migration, though he was a little puzzled with the many hibernation stories. In 1780 an anonymous pamphlet—"A Discourse on the Emigration of British Birds," flouted the theory of winter sleep in no measured terms (33). This pamphlet was, at first, attributed to George Edwards, and the 1811 edition has his name on the title, but Mr A. C. Smith shows that the real writer was a comparatively unknown man, John Legg. Legg must be looked upon as one of our first real students of migration. It is Legg who refers to a pamphlet which appeared in 1740 in which it was seriously argued that swallows migrated annually to the moon.

All this time, from 1736 onwards, the family of Marsham in Norfolk, had been quietly recording observations on the arrival of migrants, each generation continuing the work. The accumulated results have been used, and will be used again, in studying the science of "ornithophÆnology."

A myth, founded on mistaken observation as well as upon mere speculation was, and to some extent still is, that the larger migrants assist the passage of the weaker ones. How else, is still asked, can weak-winged species cross the sea? It was an old legend when J. G. Gmelin heard it from the Tartars in 1740; each crane they told him took a corncrake on its back. There are men who know the corncrake well, who believe to-day that the bird must skulk unseen through the winter, for they assert it is quite incapable of lengthy flight. It is useless to argue with them; the only answer is that it not only can, but regularly does perform a long double journey; its range extending from northern Europe to South Africa. In 1911 I handled a water-rail, a bird with short rounded wings like those of the corncrake, which had struck the lantern of a lighthouse with great violence. Its smashed head was nearly severed from its body.

Herr Otto Herman's "Recensio critica automatica" (31) supplies much information about the literature on bird migration, and the strange divergence of opinion on nearly every point. It is carried up to the beginning of the twentieth century, but much of the valuable work done in America is altogether neglected.

A short bibliography is given at the end of the present volume, including the more important works on the subject and a few of the papers in periodical publications referred to in this manual.