Abstract

In late winter of 1864, Charles Darwin received two folio volumes on radiolarians, a group of one-celled marine organisms that secreted siliceous skeletons of unusual geometry. The author, the young German biologist Ernst Haeckel (fig. 1), had himself drawn the figures for the extraordinary copper-etched illustrations that filled the second volume.1 The gothic beauty of the plates astonished Darwin (fig. 2 ), but he must also have been drawn to passages that applied his theory to construct the descent relations of these little known creatures. He replied to Haeckel that the volumes "were the most magnificent works which I have ever seen, & I am proud to possess a copy from the author."2 Emboldened by his own initiative in contacting the famous naturalist, Haeckel, a few days later, sent Darwin a newspaper clipping that described a meeting of the Society of German Naturalists and Physicians at Stettin, which occurred during the previous autumn. The article gave an extended and laudatory account of Haeckel's lecture defending Darwin's theory.3 Darwin immediately replied in his second letter: "I..