Schleicher had never heard of Darwin before Haeckel brought him to Schleicher's attention. He had published his own work on the "Stammbaum" in an article of 1853, six years before the first edition of "Origin of Species" in 1859. The concept of descent of languages was by no means new. Thomas Jefferson, a devout linguist himself, had proposed that the continual necessity for neologisms implies that languages must "progress" or "advance." These ideas foreshadow evolution of either biological species or languages, but after the contact of Schleicher with Darwin's ideas, and perhaps Darwin's contact with the historical linguists, Evolution and language change were inextricably linked, and would become the basis for classification. Now, as then, the main problems would be to prove specific lines of descent, and to identify the branch points.