302 
ROUTES OF MIGRATION. 
The Routes by wliicli dispersal has probably been chiefly accom- 
plished from the assumed evolutionary centre in North Central 
Europe, in which the highest forms of life are found and to which 
area the British Isles are immediately adjacent, is shown by the 
accomi)anying map, which indicates that the chief route for the 
occupation of Asia and America was by the narrow and comparatively 
fertile region lying to the north of the Central Asian desert and 
mountain plateau ; on reaching the Pacific shores a furcation has 
taken place, one branch crossing by an Aleutian bridge to North 
America and spreading southward to the west of the great mountain 
chain, and eventually occupying the entire continent, while the second 
contingent travelled southwards, occuj)ylng China and passing into the 
IMalayan Archipelago and Australasia or to the west towards India. 
An important stream of colonists has also left North Central 
Europe, occupying France and the British Isles and ])assing across or 
around the mountain chains limiting the North Central European 
region to the south, and have occupied the Iberian, Italian and Balkan 
peninsulas and Asia Minor, crossing by the ancient land bridges to 
Africa and eventually finding their way by the Nile valley to the 
south of the vast Saharan barrier and opening up to the immigrants 
the entire African continent, while the Caucasian migrants more 
especially press towards the east by way of Persia and Afghanistan, 
more slowly overrunning the arid and elevated country to the north 
of Persia and to the east of the Caspian Sea. 
The inclement northern region, the more inhospitable parts, of the 
great Central Asian plateau and other similar districts become more 
slowly overrun by the comparatively weaker races, forced aside by the 
tide of stronger immigrants, and in these more inhospitable districts 
the relatively weaker races comjielled in turn to be their occupants are 
successively dominant and by adaptation or specialization become 
more suited b.) the harder conditions of life to which they are e.xpo.sed 
or become e.xtinct. 
That the routes described probably broadly represent the jiaths 
traversed in turn by the various improved races, as they became 
developed within the limits of the chief evolutionary area, is strikingly 
indicated by the distribution of the Helicidre, whose characteristic 
assemblages serially representing the different stages of i)rogress 
towards the most perfected type of Helicidian organization, are distri- 
buted over the globe in strict accordance with the relative perfection or 
