32 CHELTENHAM COLLEGE NATURAL HISTORY SOCIETY. 
I should like to employ the time I have at my disposal with a 
few remarks on a part’of the subject that is very interesting to me. 
—The migratory habits of birds. 
Formerly believing that such birds as swallows flew due North and 
South, some theorists tried to account for it by supposing the bird to 
be in some way influenced by the magnetism of the earth. Even now 
some still hold a modification of this theory. Many other theories 
have been suggested, which on a careful examination have proved 
untenable, and even now our facts are not sufficient to warrant 
a definite conclusion. The facts are these.—Very few birds remain 
in the same country all the year round. They migrate on the 
approach of cold. Some, as the swallow and nightingale spend the 
Summer in England, but Winter in the South of Europe or North 
Africa. Other birds nest in colder regions and come to us as Winter 
visitors, as the fieldfare from Finland, Scandinavia, and North 
Russia. The third class, the true birds of passage, only rest in 
England in their journeys North and South. All birds lay their eggs 
and bring up their young in the coldest country to which they 
migrate. Change or failure of food supply or change of climate seem 
to be the causes which impel migration, though the migration takes 
place in advance of such change. 
Wallace’s theory of the origin of the habit is now generally 
accepted. It is this.—Thousands of years ago in the Period which 
we call the Pliocene the climate of Northern Europe was not only 
much warmer than at present, but it was also more equable, that is 
there was no great difference between Summer and Winter tempera- 
ture. Under these conditions there would be no need for birds to 
travel in search of food. As the climate changed and Summer and 
Winter temperature became more unlike so birds would have to 
follow their food supply further and further South, at first over shorter 
distances, and as the difference between the Summer and Winter 
temperature became greater over longer and longer distances. During 
a part of the Pliocene period we have proofs that the level of the 
land in parts of Furope was higher than at present and that the North 
Sea and the Mediterranean, if they existed at all, were much smaller 
than at present. It was during this Period that the birds acquired 
the habit of travelling greater and greater distances, and when the 
level of the land subsided the route was still continued. 
As a support for this theory we find that birds do actually cross 
these seas over shallower regions and where there are islands as from 
Italy to Sicily and Malta to Africa or from Greece by Crete. Apart from 
