The geographical distribution of animals, with a study of the relations ...

chap, ii.] BIRDS. 27preceding spring, whereas those which went away in autumnwere two or three times as numerous. Those young birds thatdo get back, however, have learnt by experience, and the nextyear they take care to go with the old ones. The most strikingfact in favour of the " instinct " of migration is the " agitation,"or excitement, of confined birds at the time when their wildcompanions are migrating. It seems probable, however, thatthis is what may be called a social excitement, due to theanxious cries of the migrating birds ; a view supported by thefact stated by Marcel de Serres, that the black swan of Australia,when domesticated in Europe, sometimes joins wild swans intheir northward migration.We must remember too that migrationat the proper time isin many cases absolutely essential tothe existence of the species ; and it is therefore not improbablethat some strong social emotion should have been graduallydeveloped in the race, by the circumstance that all who forwant of such emotion did not join their fellows inevitablyperished.The mode by which a passage originallyoverland has beenwithout apparently stopping on the way isconverted into one over the sea offers no insuperable difficulties,as has already been pointed out. The long flights of some birdsthought to be inexplicable,as well as their finding their nesting-place of theprevious year from a distance of many hundreds or even athousand miles.But the observant powers of animals are verygreat ; and birds flying high in the air may be guided by thephysical features of the country spread out beneath them in away that would be impracticable to purely terrestrial animals.It is assumed by some writers that the breeding-place of aspecies is to be considered as its true home rather than that towhich it retires in winter ; but this can hardly be accepted as arule of universal application. A bird can only breed successfullywhere it can find sufficient food for its young ;and thereason probably why so many of the smaller birds leave thewarm southern regions to breed in temperate or even cold latitudes,is because caterpillars and other soft insect larvae arethere abundant at the proper time, while in their winter home the