Ernst Haeckel and the philosophy of sponges

Nearly 150 years ago, Ernst Haeckel published a three volume monograph on the calcareous sponges. These volumes contained the results of his extensive investigation of the anatomy, reproduction, and development of these marine invertebrate organisms. This paper discusses how Haeckel's contribution to spongiology was so distinct from that of earlier writers on the natural history of sponges, by focusing on his "philosophy of sponges." This included "an analytic" proof of Darwin's theory of descent, an argument for the monophyletic origin of the Metazoa from an ancient sponge-like embryo (the "gastraea theory"), and proof of the philosophy of monism that humans are no different than lowly sponges in their perfectly natural and material origins according to the laws of ontogeny in a universe devoid of supernatural beings or purpose. Haeckel was a philosopher using the methods of natural science. He was also a gifted artist-as his illustrations attest-and like most artists he disliked criticism of his creations, including his theoretical work. His observations and speculations regarding sponges (and certainly his more philosophical conclusions drawn therefrom) were and continue to be criticized, but as a review of the current literature shows, Haeckel's imprint on sponge biology is still very evident.