Allen on the Instinct of Migration. 151 



ORIGIN OF THE INSTINCT OF MIGRATION IN BIRDS. 



BY J. A. ALLEN. 



Among the few who have ventured an explanation of that "mys- 

 teiy of mysteries," the migration of birds, is Mr. A. R. Wallace, who, 

 in the following passage, published six years since in " Nature " 

 (Vol. X, p. 459), seems to have suggested a clew to its probable solu- 

 tion. What he says is so pertinent that I prefer to take it as a text 

 from which to enlarge on some points here first suggested, and others 

 that seem not so clearly to have occurred to the author in question.* 

 Says Mr. Wallace : — 



" It appears to me probable that here, as in so many other cases, * sur- 

 vival of the fittest' will be found to have had a powerful influence. Let 

 us suppose that in any species of migratory bird breeding can as a rule be 

 only safely accomplished in a given area ; and further, that during a great 

 part of the rest of the year sufficient food cannot be obtained in that area. 

 It will follow that those birds which do not leave the feeding area at the 

 proper season will suffer, and ultimately become extinct ; which will also 

 be the fate of those which do not leave the feeding area at the proper time. 

 Now, if we suppose that the two areas were (for some remote ancestor of 

 the existing species) coincident, but by geological and climatic changes 

 gradually diverged from each other, we can easily understand how the 

 habit of incipient and partial migration at the proper seasons would at last 

 become hereditary, and so fixed as to be what we term an instinct. It will 

 probably be found that every gradation still exists in various parts of the 

 world, from a complete coincidence to a complete separation of the breed- 

 ing and the subsistence areas ; and when the natural history of a sufficient 

 number of species in all parts of the world is thoroughly worked out, we 

 may find every link between species which never leave a restricted area in 

 which they breed and live the whole year round to those other cases in 

 which the two areas are absolutely separated. The actual causes that de- 

 termine the exact time, year by year, at which certain species migrate will 

 of course be difficult to ascertain." 



The premises here laid down are avowedly suppositional, and the 

 hypothesis based thereon is therefore necessarily highly tentative. 



* It is due to myself to state that the ideas here briefly presented were writ* 

 ten out at greater length for use in another connection before tlie existence of 

 Mr. Wallace's remarks here quoted, which I perceive are based primarily on the 

 same fundamental conception, was known to me. 



