Autonomous states come together to form a union called

economic and political activities. As a big city population grows, so does the size and space. Thus joins other contiguous cities, through a process called coalescence.

Megalopolis: This term refers to a large urban area formed by the union of two or more metropolitan areas. It was introduced by the French geographer Jean Gottman in 1961, to refer to the agglomeration formed by the union of several metropolitan areas in the northeastern United States.

Migration: This mobility can be classified in different ways, depending on the objectives and reasons: international migration, rural-urban migration, movement of tourists, forced migration and political conflicts, among others.

Among the last decades of the century 19 (XIX) century and the first 20 (XX), as people were getting their independence, new immigrant flows were populated rural and urban sectors. Among the causes that led these migratory flows are, first, hunger, joblessness and political persecution in vast areas of Europe and, on the other hand, policies of new U.S. Government to promote immigration.

The concentration of population: In North America, 81 of every 100 people live in cities, while in South America and the Caribbean, the proportion is 78 of every 100 people.

In the second half of the 20 (XX), many of these urban spaces were converted into large clumps, which concentrated the majority of the people who inhabit the continent. Mexico City, Sao Paulo and New York, for example.

This phenomenon can speak of a geographic trend towards concentration of population in cities of different sizes.