^°190?^1 ^^^^^^ Literature. 329 



RECENT LITERATURE. 



Walter on Bird Migration/ — After presenting several pages of general 

 comment, the author proceeds to treat of bird migration mider the two 

 questions, "I. How do Birds find their way in Migration?" and "II. 

 Why do Birds migrate?" under which he re^dews, in the main to condemn, 

 various theories that have been put forth in answer, and announces as his 

 'conclusion': "There still remains an immense halo of mystery around 

 bird migration because there are so many things we do not know. We 

 not only do not know why birds migrate but as yet we do not know how 

 they migrate except in a general way." 



Under the first question he properly condemns the "instinct theoiy" 

 as a confession of ignorance. The "magnetism theory" of von Midden- 

 dorff and the "semicircular canal theory" of Mach-Bruer are both found 

 wanting, the latter ha^^ng been thorougUy refuted by the experiments of 

 Exner upon pigeons. He deals a little more kindly with the "sense of 

 direction theory," but as it lacks the demonstration of a "physical basis," 

 he deems it "is hardly better than the instinct theory since it gives the 

 answer to the problem in unknown terms." 



The "landmark theory," he states, has rather more to recommend it. 

 Thus, he says: "Exner came to the conclusion that carrier pigeons find 

 their way home by seeing familiar landmarks and when such landmarks 

 are not visible the birds explore vmtil landmarks are foimd. This explains 

 how his pigeons, whether whirled, galvanized or narcotized, were quite as 

 weU able to get home as those which had not midergone such interference 

 with their sensory impressions upon the outward journey." But he adds: 

 "The objection must be raised to the landmark theory, however, that 

 many birds do not follow river valleys, coast lines or moimtain chains in the 

 way they might be expected to do if they were gmded by what appear to 

 us to be the most obvious landmarks." It does not follow, however, that 

 because birds do not always follow river valleys or mountain chains, bvit 

 pursue courses more or less divergent from them, that they do not ser^-e 

 them as landmarks for their journeys. The recognition of such land- 

 marks would be sufficient for their guidance whether their lines of migra- 

 tion are parallel or more or less oblique to the general trend of mountain 

 ranges or river valleys. 



He concludes that in the case of carrier pigeons "the successful indivi- 

 duals are those who have been trained over the course, that is, those who 

 have learned the way either by seeing landmarks for themselves or by fol- 

 lowing a trained companion. There is no mysterious sixth sense of direc- 

 tion, no crossing of imaginary magnetic lines, no intricate automatic 



^Theories of Bird Migration. By Hubert Eugene Walter, Ph. D., Brown Univer- 

 sity. Reprinted from 'School Science and Mathematics,' April-May, 1908. 8vo., 

 16 pp., without pagination. 



