The Signature of All Things (Signatura Rerum) (1621) by Jacob Boehme

Jacob Boehme (in German, Böhme, 1575-1624) was a cobbler, mystic, and philosopher who developed a theosophy independent of the orthodox Catholicism or Protestantism of his day. He conjectured an interconnected natural world where all things carried a hidden meaning or signature, the imprint of a divine creator. Boehme was considered radical, but his ideas went on to influence later mystical movements like Quakerism, cultural movements like German Romanticism, and philosophers including Hegel, Schopenhauer, Nietzsche, and Jung.

On the Origin of Species (1859) by Charles Darwin

This links to the text of the 1859 first edition available at Project Gutenberg. Beginning in 1836, Darwin published over 20 major works, including his observations from his voyage with the HMS Beagle (published between 1838 and 1843) and The Descent of Man (1871), in which Darwin postulated that humans descended from an ape-like ancestor. On the Origin of Species, the book in which Darwin formulated his groundbreaking theory of evolution, was the culmination of over 20 years of study and observation. In describing the principle of natural selection, or “survival of the fittest,” Darwin first convincingly articulated a provable mechanism by which species mutate, evolve, and adapt.