Check out this site, especially the bio and the plates available here:
Online Exhibition of Ernst Haeckel's Art Forms in Nature
Haeckel's illustration of the developing embryos are still standard in many text books, though the accuracy of some of the features has been called into question. Next to Huxley, Haeckel was Darwin's most important popularizer, though Haeckel's views would have been more aligned in general with Spencer than Darwin. Haeckel's Monists had connections with the German Thule Society, which will be mentioned later in the readings.
His illustrations are discussed in the evolution debate that we will be reviewing as well.