MIGRATION OF BIRDS, 143 
abroad yielded the facts that led to the giving up of the first 
theory of what is now called migration. 
But travelling in foreign parts, that exploded the Hiber- 
nation Theories, brought into notice new difficulties, At 
Gibraltar, for example, it was noticed, during February in 
particular, vast numbers of our common birds flocked across 
in continuous lines from the continent of Africa: familiar 
swallows, familiar warblers, familiar birds of prey, were all 
seen in the course of a single day, where a month, or even 
a week, previously not one was to be seen. It was the 
Same in the valley of the Nile, but to a more wonderful 
extent. The huge Sahara desert is a barrier that birds do 
not care to cross over. Even our temperate region birds 
that winter along the Guinea coast, and that might, as we 
think, easily at migration times follow the west coast of 
Africa northwards or strike right across the Sahara, do not. 
Instead, they strike east till they mark the sources of the 
Nile, and then force northwards down the Nile valley. 
This week every bush along the river-side is filled with 
small birds that had not been seen for several months, and 
by another week all will have disappeared, not to be seen 
again till the month of September. The same phenomenon 
came across your vision, whether you stood in the valley 
of the Mississippi or in the valleys of the rivers that pour 
their waters into the Indus or Ganges. The phenomenon 
was to an extent world-wide. There must be a reason to 
account for this grand ebb and flow of bird-life. But what 
was the reason or underlying principle ? 
Love of home was given as the cause. These northern 
regions formed the ancestral home, and from these the 
feathered inhabitants were driven out by the awful rigours 
of the glacial periods or by the inclemencies and 
desolations of the winter season. When the warmth 
of spring came again, the desire to return home became 
the pervading passion: hence migration. Others said 
the regions more or less near to the Equator were the 
ancestral homes, and, as might be expected, the regions 
became congested, and hence the necessity for an emigration 
either north or south for a longer or shorter period of the 
year. When food-supplies were exhausted by wintry 
