148 Proceedings of the Royal Physical Society. 
summer or at their winter quarters. The great “general 
migration flight” is performed, as Herr Gatke tells us, irre- 
spective of great river-valleys crossing the line of the migrants’ 
aerial path. I conceive the great reasons for this to be that 
the birds have not nearly accomplished their journey nor 
reached their winter quarters on an east to west migration, 
and also that their requirements of rest or food have not 
necessitated their descent. 
Moreover, the rivers east of the Baltic and White Sea— 
as travellers in North-east Europe and Siberia know—have 
higher banks on their eastern or right banks than on the 
western or left banks, and on their passage westwards the 
birds have no serious obstructions to surmount, when the 
full swing of the grand migration is developed, until the 
White Sea for some, the Baltic for others, and the backbone 
of the Scandinavian peninsula for yet others, are encountered, 
a few only passing round the North Cape. 
For instance, as Giitke tells us, the Richard’s Pipits annually 
visit and rest upon Heligoland in considerable numbers, and 
their breeding haunts are known to be east of Loch Baikal. 
We must take into consideration also certain circumstances 
of climate and general suitability, as it is unreasonable to 
expect a species, in making its pioneer advances, to select 
localities which are unsuitable to its habits and food supplies 
—at least during its earlier movements. Later, when areas 
become crowded or over-populated, then and then only would 
it be reasonable to look for an expansion to areas of less 
suitable nature; and then and then only could we expect to 
find any species begin to undergo modifications in habits or 
food to suit new descriptions of locales and circumstances, 
When this later stage is reached, the next development takes 
place—the “selection of the fittest” in this struggle for 
existence and extension of range, or, otherwise, the extirpa- 
tion of the extending lines. 
That this extension is the result of special instinct or 
» effort on the part of the birds themselves I cannot admit. 
This impulse is engendered by outside circumstances, or 
environing causes equally affecting the first pioneers and 
their after- followers. This I have illustrated before in 
