1900-1920: In 1912, Sigmund Freud delivers a speech before the London Society of Psychical Research detailing for the first time his theories on the unconscious as a repository of thoughts repressed by the conscious mind. Over the next few decades, psychoanalysis grows in popularity, with thousands of psychiatrists undergoing and then practicing Freudian psychoanalysis.

Today: Though academic interest in Freud remains strong, very few practicing Freudian psychoanalysts remain.

1900-1920: World War I is fought between 1914 and 1918, resulting in tens of millions of casualties.

Today: In 2001, terrorists kill more than 3,000 people by flying jet airplanes into the twin towers of Manhattan's World Trade Center, and President George W. Bush of the United States declares war on terrorism.

1900-1920: In 1917, the world's first mass-produced tractor, the Fordson, is introduced, and farmers quickly produce crop surpluses.

Today: Governments of the United States and Britain regularly offer subsidies to...