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THURSDAY, DECEMBER 22, 1892.

MR. C. DIXON ON BIRD-MIGRATION.

he Migration of Birds: an Attempt to reduce Avian Season-Flight to Law. By Charles Dixon. (London: Chapman and Hall, 1892.)

MONG prevalent fallacies there are few more mischievous than that which holds a man to be an thority on a subject because he has written a book bout it. If the subject be one concerning which the cientific hold divers opinions, or even hesitate to deliver n opinion at all, so much is to the good of such an uthor, for he will be able to pose all the more securely a the character of a savant-though after all that only ignifies a "knowing one." If the author can boast of ome two, three, or even half-a-dozen works already pubished, the fallacy becomes almost insuperable, notwithstanding that in zoological works of a popular nature, it is scarcely too hard to say that those who write the most know the least. Nevertheless it remains the duty of the conscientious reviewer to be instant in season with his protest against this general confounding of author with authority. We have read several of Mr. Charles Dixon's works, but hitherto we have been so fortunate that we have been able to keep in petto the judgment we have formed of them. It is not given, however, even to reviewers to struggle against fate, and it has been ordained that we should have to criticize his recent volume, the title of which may be read above. To the first sentence of his preface" There is no branch of Ornithology more popular than that which treats of the Migration of Birds" -we offer no strong objection, and rejoice that there is one spot of ground, be it never so small, that we may occupy in common; but (woe it is!) that here we must part company, for the very next sentence contains a statement which we would willingly let pass as a harmless exaggeration, were it not intensified by the words which follow-and "after that, the dark"!

Mr. Dixon's acquaintance with the subject he has selected is shown by the beginning of his second paragraph-"Notwithstanding the immense popularity and importance of Migration, strange as it may seem, no work has hitherto been devoted expressly to its discussion." He is therefore not aware of the essays of Schlegeland of Marcel de Serres, which (whatever we may now think of them) were in their time "crowned" by the scientific society that published them, and though he straightway proceeds to name the works of Professor Palmen and Herr Gatke, it is to complain of them that they have only dwelt upon a portion of the subject." Far be it from us to say that Mr. Dixon has not read their works, but really there is nothing to show that his knowledge of them is more than may be picked up from the extracts which have been translated into English and published in this country, or that he has read them to any purpose that of Herr Gätke especially, because, when further on (pp. 181-186) he comes to deal with it more particularly, he regards it as if it were a mere record of captures or reputed captures of birds in Heligoland, speaking of it with contempt, and the original and rather peculiar views on migration of its author are passed

over in silence, as though they were utterly unknown to him. Mr. Dixon states that he is "equally cognizant of the researches of Weissemann" (sic) and others, which, except that Dr. Weismann, we think, would deny his having made any, we do not take upon ourselves to gainsay, though our older writers are utterly ignored, and we have a shrewd suspicion that the anonymous author of the "Discourse on the Emigration of British Birds,” published at Salisbury more than one hundred years ago, was, from actual observation, more familiar with the main facts than Mr. Dixon is-all flourishes about "avian fly-lines" and "season-flight” notwithstanding— and therefore would have been more competent than he "to bring our knowledge of Migration within the limits of order or to reduce it to Law."

Now this is exactly what in our opinion Mr. Dixon has not done. What the "Law of Migration," of which we read he re and on the title-page, may be it passes us to dis cover. The phrase is full of sweetness, but its elucidation, if we may say so, fails in light. So also is that about bringing our knowledge "within the limits of order," though that may be here taken to mean a dissertation within the limits of 300 pages or thereabouts containing something on the origin and descent of birds, a good deal about the precession of the equinoxes and the eccentricity of the earth's orbit, but still more about glacial epochs. Concerning the "Law of Migration" it is pointless. Let our author at once speak for himself in what seems to be a sort of summary of his faith, though it is long and not reserved to the end of his volume :

"We will now conclude by following in detail the migration of some single species, say from its Post-Pliocene glacial initiation to the present day, in order clearly to demonstrate Why the habit [of migration] has been acquired, and How it is practised.

"We will select the Spotted Flycatcher (Muscicapa grisola) for the purpose. It is one of our best known summer migrants, and one whose present geographical distribution admirably illustrates the phenomenon of Migration. When the Sub Polar regions of the northern hemisphere last enjoyed a warm, almost semi-tropical climate one of the mild periods of the Glacial Epochthe Spotted Flycatcher inhabited in one unbroken area the Arctic woodlands from the Atlantic to the Pacific. nocturnal during the Polar night; food was abundant; Probably it was a resident species becoming partially its conditions of life were easy, and it multiplied apace, and became a dominant, firmly established species during the thousands of years that it dwelt in this Sub-Polar habitat. So matters continued until the slow precession tricity of the earth's orbit, began to have a marked inof the equinoxes, in conjunction with increasing eccenfluence on the climate, and gradually the fair forests and the verdant plains were devastated by the ever-increasing cold. Age after age the Spotted Flycatcher was driven slowly south; summer after summer grew colder and shorter, the periods of Polar darkness more severe. At last matters became so serious that the birds began to leave their northern haunts in autumn, probably because their food became scarce as the various insects either retreated south or began to hibernate. Further and further southward these annual journeys had to be taken, until the Flycatcher at last found its way during winter into Africa, Persia, Arabia, India, China, and even the Philippines and the Moluccas. Summer after summer the belt of breeding-ground became wider and wider, and vast numbers of individuals became separated from

the rest of the species by the lofty mountain ranges, the deserts, and other physical barriers, which would effectually assist a forest or woodland haunting species. More and more severe became the winters, longer and longer; the glaciers descended lower and lower, exterminating or driving before them all living things. At last the Spotted Flycatcher, or the form which then represented this species, came to be divided into two enormous colonies-an African one and a Chinese one-the individuals of each being completely isolated from each other, summer and winter alike. During the ages that this state of things continued, the Flycatchers became segregated into two species, owing primarily to the absence of any intermarriage; the eastern race became smaller, the tail shorter, and the breast-streaks broader; or the western race became larger, with a longer tail and narrow breast-streaks. It is almost impossible to say which form now most closely resembles the ancestral species; but such are the present differences between the two races known to ornithologists respectively as Muscicapa grisola (the Western and British form) and Muscicapa griseisticta (the Eastern form). Such was the state of things at the close of this Inter-Glacial Period.

"Then came the gradual immigration north again, as precession and lower eccentricity initiated a milder climate. Age after age the journey in the spring became longer. Certain routes to and fro came to be recognized highways of passage; and so imperceptibly did the northern breeding grounds expand that the birds became regular migrants, looking upon the movement north to higher and cooler latitudes each spring as an undertaking never to be missed. Warmer and warmer became the southern haunts, stimulating and widening migration flight to the cooler temperatures prevailing near the edges of the retreating glaciers, where a suitable breeding climate could only be found.

"Let us confine our attention solely to the birds that bred in the British Islands. In the Præ-Glacial ages this area formed part of Continental Europe; a rich and fertile corner, abounding in insect life, full of haunts the Flycatcher loved. After the banishment of its race and the exile of its ancestors in Africa, the northern journey at first did not extend further than the edges of the glaciers on the Mediterranean coasts of Europe. But as these disappeared, and a warmer climate began to prevail in higher latitudes, the annual summer flight was increased. Every century the northern breeding range had increased, creeping slowly across France; higher and higher with the growing vegetation; nearer and nearer to the haunts of old. During the slow, gradual elevation and submergence that isolated Albion from 'the rest of Europe during Post-Glacial time, the regular spring journey across the sea became wider and wider; but with the intense and inherited love of home in their tiny breasts, the individuals that were born and bred in this district never failed to return each year. For 60,000 years or more has this species now crossed the sea, returning every season, not only to our islands, but each pair of individuals, as long as they live, come back to the exact locality of their previous nests. This long journey, gradually growing longer and longer during thousands of years, until it is now at least a thousand miles in length, has grown to be a deeply-rooted custom sanctioned by the practice of ages of experience and need, and looked upon now as part of the Flycatcher's very existence ! " (pp. 58 62).

This, we think, is Mr. Dixon at his best, and we are anxious that our readers should so see him. He goes on to call it a "thoroughly demonstrable instance," which shows what his idea of a demonstration is. We do not deny that all may have happened as he here prescribes,

but who knows that it did? To begin with, we ma what proof is there of the existence on the earth it Muscicapa grisola "when the Sub-Polar regions = northern hemisphere last enjoyed a warm, almos: tropical climate"? That its ancestors then lived not doubt, but who can tell us what they were What is meant by its "becoming partially not during the Polar night"? If so its eyes must sittet undergone a considerable change, and that would h be unattended by a corresponding change in other the bird's structure. But still it is a pleasing sugge that "its conditions were easy" in those millen and we hope Mr. Dixon may be right, though for r part we cannot help fearing that the struggle for exce must have already begun. Certainly it set in at ka those terrible glaciers drove the poor bird before t with the effect-Mr. Dixon, we think, is to blame t giving us the geographical details (which of course be known to him) of the process-of dividing the st or the form which represented it, and may be pre (though this is not mentioned) to have by that tize rid of its owls' eyes, "into two enormous colosit-African one and a Chinese one." These were so is that inter-marriage between the individuals of the portions was impossible, the remarkable consequa which was that "the Eastern race became smaller the Western-a character distinctive indeed of the races, the Pygmies excepted, now inhabiting the lands-but with "the tail shorter "-a contradi character, since the long tail of a Celestial is the important part of him. We are also told that almost impossible to say which form now most resembles the ancestral species," an unexpected conte of ignorance (the "almost" is good) after so much mation, but one to which we see the necessity of However, what is the upshot of all this? And how? "law" illustrated by it? Setting aside the vag which we have just commented, it reads to us as merely an amplification of suggestions that were tively and cautiously submitted in these colum than eighteen years ago (NATURE, vol. x. pp. 416 and. The partiality of birds for their old homes was the (so far as we know) for the first time, pointed out possible factor in establishing migratory habit; another (and equally for the first time), the growing gence of breeding and feeding areas through causes was briefly and clearly set forth by Mr." Notwithstanding Mr. Dixon's assertions, he does to have advanced the question one bit, but he whelmed it in a flow of words with a great deal” and apparently always will be, incapable of proof and elsewhere throughout this volume we are br face one of that school of biologists, the growth years, which may be called the Assertive. I respects it is a very nice one to join. You have a to say what first comes into your head, goes well. Everybody that differs from you is a some extent this school resembles that Dog which a few naturalists here and there still t inasmuch as the dissentient from either was t with the same contempt. The Dogmatists have: day, but if we look back upon their doings, that in most cases they had something to go up

we 5