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- 
tery 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 the existence of 
Mr. Wallace’s remarks here quoted, which I perceive are based primarily on the 
same fundamental conception, was known to me. 
