556 MIGRATION 
other hand we must remember what has above been advanced in 
regard to the pertinacity with which Birds return to their accustomed 
breeding-places, and the force of this passionate fondness for the 
old home cannot but be taken into account, even if we do not 
allow that in it lies the whole stimulus to undertake the perilous 
voyage. 
Mr. Wallace in some remarks on the subject (Nature, x. p. 459) 
ingeniously suggests the manner in which the habit of Migration 
has come to be adopted : — 
‘« Tt appears to me probable that here, as in so many other cases, ‘survival 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 accom- 
plished 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 breeding 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 geo- 
logical 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 breeding 
and the subsistence areas ; and when the natural history of a sufficient number 
of species 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.” 
A few more particulars respecting migration are all that can 
here be given, and it is doubtful whether much can be built upon 
them. It has been ascertained by repeated observation that in the 
spring-movement of most species of the northern hemisphere the 
cock-birds are always in the van of the advancing army, and that 
they appear some days, or perhaps weeks, before the hens.? It is 
not difficult to imagine that, in the course of a journey prolonged 
throughout some 50° or 60° of latitude, the stronger individuals 
should outstrip the weaker by a very perceptible distance, and it 
can hardly be doubted that in most species the males are stouter, 
as they are bigger than the females. Some observers assert that the 
same thing takes place in the return-journey in autumn, but on 
this point others are not so sure, which is not surprising when we 
1 Jn principle Capt. Hutton (Zrans. New Zeal. Inst. 1872, p. 235) had 
already foreshadowed the same theory, which some writers have called that of 
‘“land-bridges.” 
2 This fact, often regarded as a very recent discovery, was made known by 
Montagu in 1802 (Orn. Dict. Introd. pp. xxviii., xxx. note), and had also been 
observed by Sheppard in 1819 (Zrans. Norf. & Norw. Nat. Soc. iii. p. 391). 
