I MIGRATION 19 
scarcely any writers on the subject have had sufficient data to 
form an hypothesis, so that it 1s not suprising that hardly any 
two agree in theory.’ In other parts of the world there is still 
less ground for theorising, though in North America many 
valuable observations have been made; and these, in conjunction 
with those carried on in Europe, will no doubt in due time lead 
to satisfactory results as regards the Northern Hemisphere. 
Concerning the Southern our ignorance is almost complete. 
Of the way in which Migration is performed there is still 
much to learn—but one thing is certain, all Birds do not migrate 
in the same manner. Some gather in flocks, great or small, others 
seem to accomplish their northward journey in pairs, or at any- 
rate arrive at their breeding-quarters already paired. Some 
undoubtedly voyage by night, others may be seen to travel by 
day. Of the Birds which in spring arrive unpaired, it is now 
incontestable that the males outstrip or precede the females. 
There is, moreover, equal diversity in the southward movements 
towards the close of summer and all through the autumn. Of 
some species the earher broods disappear without attracting 
attention, and the later broods as well as the parents slip away 
almost as imperceptibly. In one remarkable case, that of the 
Cuckoo, the adults leave this country long before the young are fit 
to follow; but, in by far the greater number, the young start first, 
and are followed, often at an interval of some weeks, by their 
parents.” It is contended by many that of actual Migration we 
see very little, since it is constantly carried on at a beight where 
the Birds are beyond our ordinary observation, and as regards 
some species this seems to be true. Moreover, it would seem 
that the longest flights are performed by night, and when the 
sky is clear, so that only in thick weather do the Birds come near 
enough to the earth to be heard—seeing them being of course 
impossible in the dark, though in a few cases they have been 
telescopically observed passing across the face of the moon. It 
is certain that many of the smaller land-birds gradually press 
' For the best collection of facts, see the various reports of the Migration Com- 
mittee of the British Association, 1880-1888 ; and especially that for 1896, con- 
taining the Digest of the observations (made at Lighthouses and Lightships) by 
Mr. W. Eagle Clarke. 
* It has been suggested that these flocks of young birds are led by older members 
of their own species which, though for some cause not breeding, have yet had 
experience of migration ; but of this there is no evidence whatever. 
