100 SECTIONAL ADDRESSES. 
of the lithosphere and of the atmosphere which are interpenetrated by 
the hydrosphere. It is obvious that a dry lithosphere and a dry atmo- 
sphere would present a landscape of merely naked rocks, not even rusted, 
and seas of sterile sand with perhaps endless lines of wind-driven dune. 
When we reflect that the millions of stars ‘ have surface temperatures of 
anything from 1650° to 30,000°’ (Jeans), what an astonishing thing it is 
that through the long geological ages the surface of our earth has enjoyed 
that short range of temperatures which will permit of the liquidity of 
water. 
I turn to biogeography, for it is there that our difficulty lies. The 
fundamental unit of biogeography is the natural region. Apart from the 
idea of habitat or environment the natural region would have no meaning. 
What gives to it significance is the fact that, whether you accept the 
inheritance of acquired characteristics, or believe in natural selection 
alone, species originate as local varieties. The African elephant is no 
doubt a single interbreeding species, but the elephant of the western 
tropical forests is of a different variety from the elephant of the eastern 
uplands. The same is no doubt true of the mountain and the forest 
gorillas. The same again is the case to an almost infinite extent with the 
varieties of the many species of butterfly. Now the basis of the fixation 
of a local variety into a species is that the variety shall consist not of so 
many individuals but of a single blood. Forgive me if for convenience I 
speak in what follows of blood instead of protoplasm. 
Let us go for an example to the head of the list: to Man himself. 
John Bull is a local variety of the genus-and species, homo sapiens. 
There are to-day some forty millions of us inhabiting a natural region 
which we may describe as the English Plain, for there are few who live 
much over 600 feet above the sea. The fact that each of us is the off- 
spring of two parents results in this: that if you go back to the time of 
the Norman Conquest each of us now living had more than sixty million 
ancestors. But in the England of that time was a population of only two 
millions. If we assume a fluid marriage market, then each of us living 
would be descended from each of the two million people of the time of the 
Conqueror by thirty different pedigrees. The assumption of fluidity in 
marriage opportunities is, of course, not quite true, but although in the 
past Lincolnshire may rarely have married Hampshire, yet Lincolnshire 
married Northamptonshire, and Northamptonshire—Oxfordshire, and 
Oxfordshire—Berkshire, and Berkshire—Hampshire. There was a social 
continuity from the Humber to Southampton Water. Thus in literal 
truth there is to-day in England a single blood, although with some 
poe thickenings. When, however, we come to the shore of the © 
nglish Channel or of the North Sea, we look across to other bloods in 
France and the Netherlands from which a few drops have splashed across 
during the centuries. A different kind of boundary delimits the English — 
Plain along the Welsh Border. When Simon de Montfort marched in 
1265 along that border to meet his fate at the Battle of Evesham, his 
English soldiers, accustomed to living on bread, became mutinous because 
their allies in the Civil War, the Welsh mountaineers, could give them to 
eat nothing but the meat and milk of the upland (Oman). Some centuries 
earlier the English blood, and with it, as it happened, also the English 
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