The break-up between Darwin and Haeckel.

In the course of the second half of the nineteenth century, following the publication of Darwin's On the Origin of Species (1859) and Haeckel's Generelle Morphologie der Organismen (1866) (General Morphology of Organisms), the two men interacted like comrades in arms, leading the theory of evolution to victory in the international arena. This relationship broke up during the early decades of the twentieth century. The cause was primarily political, not scientific, and was brought about by the nationalistic mobilization of scientists that accompanied WW I and the Russian Revolution. In the course of these military and political upheavals, national flags were wrapped around different approaches to evolutionary biology. Darwinian natural selection became Anglified, Haeckelian morphology was Germanified, and in the Soviet Union, a Marxist version of Darwinian theory took root. The process of break-up was further affected by the emergence of Nazi ideology.