Karl Marx was born in the German city of Trier in 1818. He studied law in Bonn and Berlin at his father's insistence, but his true interests lay elsewhere and, in 1841, he received a doctorate in philosophy from the University of Jena.

For the next two years he wrote for radical left-wing newspapers before moving to Paris with his wife, Jenny; there he became a communist and met his lifelong friend and collaborator, Friedrich Engels.

They published their revolutionary pamphlet, The Communist Manifesto, in 1848 and Marx moved to London a year later. He spent the rest of his life there - often in considerable poverty - while he wrote his magnum opus of political theory, Das Kapital. Karl Marx died in 1883.

Henry David Thoreau (1817-62) was born in Concord, Massachusetts and educated at Harvard. He became a follower and a friend of Emerson, and described himself as a mystic and a transcendentalist.

Although he published only two books in his lifetime, Walden is a literary masterpeice and one of the most significant books of the nineteenth century.

Charles Darwin (1809–19 April 1882) is considered the most important English naturalist of all time. He established the theories of natural selection and evolution.

His theory of evolution was published as On the Origin of Species in 1859, and by the 1870s is was widely accepted as fact.