152 REV. W. SERLE ON 
them their natura] food. In the end the tendency would be 
for birds to differentiate—and that, if they were constrained 
to take to the habit of migration. Not all would migrate, as 
I said at the beginning. Some, where winter had little 
sway, would become fitted with certain qualities which would 
give them in these regions a decided advantage over those 
that had to migrate for a period of the year. The lapse of 
time would still more specialise these birds, and if they 
became isolated at certain centres they would also become 
highly specialised. It is this, no doubt, that has accounted 
for the number of peculiar forms on the islands of tropical 
and sub-tropical seas: witness the many peculiar forms of 
birds in the islands lying round Borneo, or in Madagascar. 
But, after all, congestion in original centres would force 
migration just as it would structural differences among birds, 
and you have to remember that the ever-changing conditions 
of the glacial periods would have effects that to-day would 
declare that certain finches had their original centre in some 
temperate region, and a vast army of waders and ducks and 
sea-birds had theirs in more northern regions. Satisfactory 
knowledge on all these points it is forever impossible to get. 
But for the migrants the natural progress of events is likely 
to have been that the first migrant moved north or south, 
according as it was the more suitable. They would move 
along the line of least resistance—that is, along the lines 
where their peculiar wants would be most easily supplied. 
Migration, or better, emigration, would tend towards distance 
from the Equator. 
Nor is this a thing that has taken place wholly in past 
ages, but is going on on a greater or less scale every year: 
perhaps more markedly now than ever, seeing that civilised 
man so easily changes the face of Nature. In our northern 
counties, where large tracts have been put under forest 
cultivation, birds have made their appearance, and promise 
to become common where they had never been seen before. 
Nor has the migration ever tended northwards exclusively, 
as is sometimes asserted; for when circumstances have been 
favourable, birds have shown a willingness to halt and breed 
in lands that they have passed over. There is the notable 
instance of the short-eared owl a few years ago nesting 
