The preponderant migration of animals from the northern to the southern hemisphere is attributed by Darwin to the greater extent of land in the north, whereby the northern types have existed in greater numbers and have been so perfected through natural selection and competition, that they have surpassed the southern forms in dominating power and therefore have encroached successfully.[310] Also the races and nations of the northern continents have seriously invaded the southern land-masses and are still expanding. It is the largest continent, Eurasia, which has been the chief center of dispersal. [Sidenote: Political domination of large areas.] The Temperate Zone of North America will always harbor a more powerful people than the corresponding zone of South America, because the latter continent begins to contract and tapers off to a point where the other at the northern Tropic begins to spread out. Therefore North America possesses more abundantly all the advantages accruing to a continent from a location in the Temperate Zone. The wide basis of the North Slavs in Russia and Siberia has given them a natural leadership in the whole Slav family, just as the broad unbroken area of ever expanding Prussia gave that state the ascendency in the German Empire over the geographically partitioned and politically dismembered surface of southern Germany. English domination of the United Kingdom is based not only upon race, location, geographical features and resources, but also on the larger size of England. So in the United States, abolitionist statesmen adopted the most effective means of fighting slavery when they limited its area by law, while permitting free states to go on multiplying in the new territory of the vast Northwest.