360 THE STRUCTURE AND LIFE OF BIRDS cHap. 
Acrocephalus turdoides, our Great Reedwarbler, is found in 
summer as far north as the south of Sweden. Its winter mi- 
gration takes it to the Transvaal and even further south. 
A. orientalis differs little except in having a slightly less 
pointed wing. It migrates from Japan to Borneo. 
A. stentorius. Wing much more rounded. It migrates only 
from Turkestan to India. 
A. syrinx has the roundest wing of all. It does not migrate, 
but is resident in the Island of Ponape. 
It is highly probable that among individuals of the 
same species similar differences exist, that those 
which travel furthest on migration are better fitted 
by the shape of their wings for long voyages than 
those which have a less extended range. This has 
actually been observed, it is said, in the case of the 
Wheatear and the Willow Wren. It is very natural 
that such variation should be found if there is truth in 
the theory that among the birds of a particular species 
those that winter furthest south pass the summer 
furthest north. Mr. Seebohm gives strong evidence 
of this. The Swallows at Natal start for the north 
the last week of March, only those that were hatched 
the previous spring setting out later, in the first half 
of April. But the swallow returns to southern 
Europe by the end of January, and in Spain Mr. 
Howard Saunders found many broods hatched by 
April 16th. It seems clear, then,,that the Natal 
Swallows do not stop till they have made their way 
further north, and that our own, which arrive in the 
first half of April, when those which stop in Spain 
are well on with their nesting, have come from the 
far south. A prior?, too, it is probable that birds 
which spend their summer in the cool northern climes 
