NATURE 



169 



THURSDAY, DECEMBER 22, 1892. 



MR, C. DIXON ON BIRD-MIGRATION. 

 The Migration of Birds : an Attempt to reduce Avian 

 Season-Flight to Law. By Charles Dixon. (London : 

 Chapman and Hall, 1892.) 



AMONG prevalent fallacies there are few more mis- 

 chievous than that which holds a man to be an 

 authority on a subject because he has written a book 

 about it. If the subject be one concerning which the 

 scientific hold divers opinions, or even hesitate to deliver 

 an opinion at all, so much is to the good of such an 

 author, for he will be able to pose all the more securely 

 in the character of a savant — though after all that only 

 signifies a " knowing one." If the author can boast of 

 some two, three, or even half-a-dozen works already pub- 

 lished, the fallacy becomes almost insuperable, notwith- 

 standing 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 ^^//(? 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 j 

 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 harm- 

 less 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 para- 

 graph — " 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 

 Palm(^n 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 know- 

 ledge 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 Gatke 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 Heli- 

 goland, speaking of it with contempt, and the original and 

 rather peculiar views on migration of its author are passed 

 NO. 1208, VOL. 47] 



over in silence, as though they were utterly unknown to 

 him. Mr. Dixon states that he is " equally cognizant of 

 the researches of Weissemann " isic) and others, which, 

 except that Dr. Weismann, we think, would deny his 

 having made any, we do not take upon ourselves to gain- 

 say, 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," pub- 

 lished 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 

 discover. The phrase is full of sweetness, but its eluci- 

 dation, if we may say so, fails in light. So also is that 

 about bringing our knovi^ledge ''within the limits of 

 order," though that may be here taken to mean a disser- 

 tation 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 migra- 

 tion 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 Epoch — 

 the Spotted Flycatcher inhabited in one unbroken area 

 the Arctic woodlands from the Atlantic to the Pacific. 

 Probably it was a resident species becoming partially 

 nocturnal during the Polar night ; food was abundant ; 

 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 

 of the equinoxes, in conjunction with increasing eccen- 

 tricity of the earth's orbit, began to have a marked in- 

 fluence on the climate, and gradually the fair forests and 

 the verdant plains were devastated by the ever-increas- 

 ing 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 



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