62 ANIMAL INSTINCT AND INTELLIGENCE 
have been determined with some approach to accuracy. A vast 
number of facts concerning migration have already been collected, 
and these receive large additions every year, so that in the course 
of time we may expect to have a fairly complete knowledge of 
the movements of many migratory birds. To discover how they 
find their way is a much more difficult problem, especially in cases 
where there can have been no previous experience. Many of 
them, eg. the Common White Stork (Czconza alba), “assemble” 
before migration, as if to practise their powers of flight, and the 
writer once saw the roof-ridges in the street of a midland town 
“lined” with hundreds of swallows at 6 a.m. one morning, less 
than two hours after which all had disappeared. It has been 
suggested that the old birds impart geographical knowledge to 
the young, and also that the migrant flocks are ‘personally con- 
ducted” by experienced leaders. But many well-ascertained facts 
militate strongly against such views, at any rate for certain species. 
Some young birds go off by themselves, even the first time they 
migrate, and this may take place under conditions which pre- 
clude the possibility of their having previously acquired informa- 
tion from their elders. The Common Cuckoo (Cuculus canorus), 
for example, winters in Africa, and, as everyone knows, its off- 
spring are reared by other birds. The old Cuckoos have all left 
this country by the end of August, and the young ones take 
their departure later. In such a case we cannot doubt the ex- 
istence of a “ migratory instinct”, but how far this is modified 
by intelligence has yet to be determined. We are equally igno- 
rant as to the sense-organs which are the agents of the instinct, 
and its possible modifications by intelligence, even more than 
we are in the cases of insects and molluscs which possess a 
keen sense of locality. We ourselves are not entirely devoid of 
a faculty of the kind, and it appears to be comparatively well 
developed in savage races. It is to be hoped that extended 
observation and experiment will some day throw more light upon 
the subject, till when it will be wise to suspend our judgment, 
and remain in a critical attitude, rather than indulge in prema- 
ture generalization. It is scarcely necessary to add that there is 
room for a host of unprejudiced observers in this field of work. 
