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 luidertake the perilous 

 voA^age. 



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 : ^ — • 



" It 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 gi-eat part of the rest of the 

 year sufBcient 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 sufler, 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 migi'ation 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 gi'adation 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 suflicient 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 joui'ney 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 sui'e, which is not surprising when we 



1 In principle Capt. Hutton [Trans. New Zeal. Inst. 1872, p. 235) had 

 already foreshadowed the same theory, which some writers have called that of 

 "land-bridges." 



- This fact, often regarded as a very recent discovery, was made known by 

 Jlontagu in 1802 {Orn. Diet. Introd. pp. xxviii., xxx. note), and had also been 

 observed by Shepjiard in 1819 {Trans. Norf. d: None. Nat. Soc. iii. p. 391). 



