"[Ernst] Haeckel was the chief apostle of evolution in Germany. Nordenskiold (1929) argues that he was even more influential than Darwin in convincing the world of the truth of evolution. Yet influential as Haeckel was among scientists, his general impact was even greater. Nordenskiold writes: "There are not many personalities who have so powerfully influenced the development of human culture--and that, too, in many different spheres--as Haeckel" (p. 505).
From the 1880's onward, he focused increasing attention on the political, social, and religious implications of his biological views--a set of ideas that he amalgamated into his "monistic" philosophy....But, as Gasman argues, Haeckel's greatest influence was, ultimately, in another, tragic direction--national socialism.
His evolutionary racism; his call to the German people for racial purity and unflinching devotion to a "just" state; his belief that harsh, inexorable laws of evolution ruled human civilization and nature alike, conferring upon favored races the right to dominate others; the irrational mysticism that had always stood in strange communion with his brave words about objective science--all contributed to the rise of Naziism. The Monist League that he had founded and led, though it included a wing of pacifists and leftists, made a comfortable transition to active support for Hitler."
Stephen Jay Gould, Ontogeny and Phylogeny, Belknap Press of Harvard University
--------
Well-read individuals know that Haeckel was one of the most prominent evolutionists in history, and still the best-known and most influential biologist in German history. His influence on Hitler and the Nazis was enormous, as Gould readily admits.