The USA takes the third place to population. Now in the USA live representatives of hundreds ethnic groups; scientists unite them into 3 groups:
1) Americans
2) Immigrants
3) Aboriginals
The USA often is called a country of immigration, and that is really so. On the one hand, the rapid expansion of the U.S. territory, its occupation and developing had created constant additional requirements in admittance of more and more human contingent. The USA takes the eighteenth place for the average population density in the world (27 people per 1 km). Almost 70% of population occupies 12% of the territory.