MIGRATION OF BIRDS. 147 
these, still there is no intense heat to dry up the very yolk 
in the eggs and cause so many to be forsaken, 
The second great difficulty in the subject was also 
presented by the increased information that flowed in upon 
us. Birds travel by certain well-known routes—yea, there 
are cross routes, and yet no mistakes are made. Birds from 
Iceland strike the north of Scotland and pass down the 
west coast, thence the east coast of Ireland, and cross over 
on to the Welsh coast. Birds from the north of Europe 
cross the North Sea, strike the British coasts between 
Shetland and Norfolk, and cross to the Continent on their 
south journey again at the Straits of Dover, Birds that 
cross the Mediterranean hug the north shores of Africa and 
pass down the valley of the Nile. And all these birds have 
their peculiar routes. Birds of certain species that breed in 
North Europe amidst the birds that Visit us in autumn 
never join these autumnal flocks, but pass south over the 
island of Heligoland towards the Frisian coasts. In these 
regions they encounter the numerous birds that are flocking 
from Central Europe westwards to England and Ireland, 
but they do not get into these flocks but keep to their own 
course. It is comparatively a rare thing for birds to get 
out of their course, and when they do, and are shot, it is 
then we read of rare birds in the public prints. How do 
these birds know the exact routes and are not influenced 
into wrong routes by passing birds? There we get the 
Theory of Tradition as stated by Darwin, Wallace, Seebohm, 
and other well-known naturalists. In every flock that 
migrates there is always a percentage of old birds that have 
been over the journey before. Birds are provided with 
good memories. When they have been over the route a 
few times they have the knowledge of the way thoroughly 
in their minds; they have extra good eyesight, and from 
the height in which they travel they have a wide view of 
the chief physical features of the land below them—they 
note the mountain ranges, the arms of the sea, the headlands, 
and by these they guide themselves till they come to their 
familiar haunts. If the difficulty is suggested that many 
birds migrate by night, still they do not choose very dark 
nights for their movements: what light there is of the moon 
