CHAP. II.] 
BIRDS. 
27 
preceding spring, whereas those which went away in autumn 
were two or three times as numerous. Those young birds that 
do get back, however, have learnt by • experience, and the next 
year they take care to go with the old ones. The most striking 
fact in favour of the “ instinct” of migration is the “ agitation,” 
or excitement, of confined birds at the time when their wild 
companions are migrating. It seems probable, however, that 
this is what may be called a social excitement, due to the 
anxious cries of the migrating birds ; a view supported by the 
fact stated by Marcel de Serres, that the black swan of Australia, 
when domesticated in Europe, sometimes joins wild swans in 
their northward migration. We must remember too that migra- 
tion at the proper time is in many cases absolutely essential to 
the existence of the species ; and it is therefore not improbable 
that some strong social emotion should have been gradually 
developed in the race, by the circumstance that all who for 
want of such emotion did not join their fellows inevitably 
perished. 
The mode by which a passage originally overland has been 
converted into one over the sea offers no insuperable difficulties, 
as has already been pointed out. The long flights of some birds 
without apparently stopping on the way is thought to be inex- 
plicable, as well as their finding their nesting-place of the 
previous year from a distance of many hundreds or even a 
thousand miles. But the observant powers of animals are very 
great ; and birds flying high in the air may be guided by the 
physical features of the country spread out beneath them in a 
way that would be impracticable to purely terrestrial animals. 
It is assumed by some writers that the breeding-place of a 
species is to be considered as its true home rather than that to 
which it retires in winter ; but this can hardly be accepted as a 
rule of universal application. A bird can only breed success- 
fully where it can find sufficient food for its young ; and the 
reason probably why so many of the smaller birds leave the 
warm southern regions to breed in temperate or even cold lati- 
tudes, is because caterpillars and other soft insect larvae are 
there abundant at the proper time, while in their winter home the 
