MIGEATIONS OF EUEOPEAN BIEDS. 325 
up on Ohristmas-d.ay_, croucliing among furze bushes, almost 
insensible from cold. The winter homes of European birds of 
passage comprehend Southern Europe, Lower Egypt, and the 
countries that he between the desert and southern shores of 
the Mediterranean, including the elevated lands of Tunis, 
Algeria, and Morocco, which, although differing in physical 
features and, in some respects, in climate, are, strictly speaking, 
but an extension of Europe, for their flora and fauna are Euro- 
pean. It is only when the traveller crosses the Sahara, with its 
salt lakes and moving clouds of sand, and gains the region of 
verdure beyond, that he enters on a new zoological and 
botanical province. It is curious and instructive to observe 
how weh this statement accords with late geological discoveries. 
Erom a series of ascertained facts the student of physical 
science is enabled to speculate on a time when equatorial 
Africa was divided from the northern portion of the continent 
by a great sea, of which the Sahara formed the bed ; it ex- 
tended from the Grulf of Gabes to Senegambia in the west, 
and was many hundred miles in breadth. The Mediterranean 
sea did not then exist ; therefore there was no great obstacle 
to the southern migrations of animals until they reached the 
shores of the great central African sea ; but as there was no 
desert in those days, there would be no hot winds to temper 
the climates northwards, and consequently we should expect 
to find traces of more rigorous winters in Central and Southern 
Europe; and such have been clearly proven by certain evi- 
dences, which were lucidly explained by Sir Charles Lyell at 
the last meeting of the British Association. Thus, although 
we may wonder at the extraordinary intelligence which prompts 
the bird to cross the Mediterranean, we see at the same time 
that it is going to no foreign land, where it will not meet 
friends to cheer it, or food unsuited to its wants. The two 
great causes which bring about the regular migrations of birds 
are either change of climate or failure of food — most often 
both combined. Any ordinary observer must have often re- 
marked that the first effect of a decrease in temperature in 
autumn is the sudden disappearance of many winged and 
wingless insects, on which many soft-billed birds of passage 
depend. At that season swallows, that seemed so full of life 
and vigour, skimming over fields, threading along the lanes, or 
twittering from straw-built sheds, are soon seen collecting in 
flocks, and flitting about with a marked diminution in their 
activity — ^now huddhng together on the eaves of houses, or 
assembling in long lines on the telegraph wires ; another 
boreal blast, not yet sufficient to turn the leaf, sends the 
whole flock southwards, for they soon find that there is no use 
facing the north from whence the cold puffs are coming, whilst 
by holding in the direction of the sun, with the balmy 
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