104 SECTIONAL ADDRESSES. 
withstanding the oceanic break it may be regarded as a single area, for 
the distance from E.N.E. to W.S.W., from the Volga to the Mississippi, 
measures only some seven thousand miles, or little more than a quarter 
way round the globe along the Great Circle. The rainfall on the land is 
drawn from the same source both in Europe and eastern North America ; 
it comes mainly from the south, from the Atlantic, and is of the order of 12 
thousand tons per human inhabitant per annum. There is an annual net 
increase of population of some four or five millions and, as compared with 
this, emigration to the outer world is small, for the movement of a million 
migrants a year from Europe to North America in the dozen years at the 
commencement of this century was, of course, internal to the area. 
Thus we have two areas, measuring together less than 10 per cent. of the 
world’s surface, but containing more than 80 per cent. of the world’s 
population. Outside is some 90 per cent. of the world’s surface, but only 20 
per cent. of the population. On some 40 million square miles of land, outside 
the East and the West, you have an average density of population of only 
ten to the square mile, as contrasted with 120 on the five million square 
miles of the West, and 180 on the five million square miles of the East. 
The moisture upon the land areas, outside the Western and Eastern rain 
zones, varies from Saharan drought to Amazon and Congo deluge, 
but it is a remarkable fact that South America, with all its fertility, has 
upon its 63 million square miles a population of only ten to the square 
mile, or the average for the world outside West and East. This fertile 
vacancy of South America may be regarded perhaps as a third great 
feature of the Habitat of Man; it must be ‘set alongside the extra- 
ordinary and persistent self-containedness of the East and the West. The 
increase ‘in the world’s population, some 12 millions a year, is mainly 
retained in its native Hast and West, and the growth of the outer popula- 
tions, even though reinforced by some immigration, is relatively insignificant. 
The main growths, the spread of the sheets of human blood, have been 
merely overflows from the anciently occupied regions into adjacent areas 
—tnto North and North-Eastern Europe, into Eastern North America, 
and into Manchuria, and in each case the natural frontiers of drought 
and frost have now been approached, except for relatively narrow 
outlets along the wheat belts of North America and Siberia. Even 
in North America the centre of population has ceased to move appreciably _ 
westward. 
In this continued growth of population in the Hast and the West 
in far greater actual number than in the rest of the world, notwithstanding 
the abundant rainfalls in several large regions elsewhere, we have an 
instance of geographical momentum. That momentum, though issuing 
from the past, is a fact of the present, an element in the dynamic system 
of to-day’s geography. I repeat an analogy which I have used elsewhere. 
If I stand on a mountain-top there are two answers to the question why 
Iam there. The first is that the rocks hold me up—that is the dynamic 
answer; the second is that I climbed there—that is the historical or 
genetic answer. Whether we look backward or forward, our genetic 
studies should start from a firm hold on the dynamic system of the present. 
The trained geographer may restore imaginatively the geographies of the 
past and so contribute to geology, archeology, and history; in a word, 
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