Resources:
http://en.wikipedia.org/wiki/Human_migration
http://en.wikipedia.org/wiki/File:Invasions_of_the_Roman_Empire_1.png
http://en.wikipedia.org/wiki/Mass_migration
http://www2.ed.gov.html
http://justschools.gsers.ucla.edu/crisis/overcrowding/index.html
Reader's Digest World Atlas ISBN 978 0 276 42724 4
