xvii Man in Nature 335 



yielding a superabundance. Western Europe has a dense 

 population on this account. Traffic, or carrying commodi- 

 ties to and fro, gives rise, at points where a change of 

 routes or means of conveyance occurs, to a local concen- 

 tration of population, and thus trading towns arise at 

 harbours, fords, and the intersections of roads or rail- 

 ways. 



428. Centrifugal Migrations. In a primitive state of 

 society the migrations of tribes are not unlike the migra- 

 tion of the lower animals, being directed from regions in 

 which the means of life no longer suffice for the in- 

 habitants. They are of the nature of evictions. A much 

 larger population formerly resided in Central Asia, the 

 margin of the Gobi ( 381) being lined with remains of 

 ruined cities ; but the desiccation of the continent drove 

 the people outward into whatever lands afforded food for 

 their cattle or plunder on which to live. The people 

 against whom the hordes of wanderers were driven were 

 in turn dispersed in all directions, and the disturbance 

 spread throughout every part of Eurasia. Overcrowding 

 in countries of dense population also necessitates migration 

 to more thinly peopled regions ; but here as a rule the 

 human power of discrimination and choice regulates the 

 resulting movement. Lands are sought out which afford 

 similar natural conditions to those in which the emigrants 

 have formerly lived, and promise an easier or more pros- 

 perous life than the overcrowded country could offer. 

 Thus the people of North-western Europe, and particu- 

 larly of the British Islands, have thronged in millions to 

 North America, South Africa, and Australia ; while num- 

 bers of the people of Southern Europe have migrated to 

 South America and Northern Africa. Another form of 

 Centrifugal Migration is the voluntary exile of people 

 persecuted for holding particular religious or political 

 opinions. The settlement of New England by the Puri- 

 tans, of Maryland by Irish Catholics, and of Utah by the 

 Mormons, illustrates the action of this principle. 



429. Centripetal Migrations have exercised an extra- 

 ordinary influence in modern times. They are the result 



