http://en.wikipedia.org/wiki/Friedrich_Ratzel
http://www.britannica.com/EBchecked/topic/492152/Friedrich-Ratzel
http://www.google.com.my/search?hl=en&q=Ellen+Churcill+Semple&btnG=Search&meta=
http://geography.about.com/od/historyofgeography/a/williamdavis.htm
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