The Riddle of the Universe (Barnes & Noble Digital Library): At the Close of the Nineteenth Century

This 1901 monograph sets forth Haeckel's philosophical conviction that the riddle of the physical universe and that of human intellect are one and the same, and must have the same answer.  A thorough going monist, Haeckel typifies a historically important point of view: that of unabashed confidence in the ability of western science to solve all of life's mysteries.