The last name along the path from Darwin to the Nazis that we need to consider is the zoologist Ernst Haeckel, Germany's best-known Darwinist and a fanatical supporter of eugenics.

In the history of science, Haeckel is known for his theory that "ontogeny recapitulates phylogeny." According to this evolutionary theory, Haeckel was claiming that embryonic development repeats "evolutionary history." He thought that the stages of embryonic development repeated the adult stages of the ancestors of a species. In order to support his theory, which he developed under the influence of Darwin, Haeckel made a number of drawings of embryos, in which, it was later realized, he had made deliberate distortions, and that his theory was a forgery. Haeckel was a charlatan who used falsified evidence to make Darwinism scientifically acceptable.

Another instance of Haeckel's erroneous science is the theory of eugenics. He adopted the theory, from such names as Charles Darwin, Francis Galton and Leonard Darwin, and took it further, by suggesting a return to the Spartan model of ancient Greece: In other words, to murdering children! In his book Wonders of Life, Haeckel proposed the "destruction of abnormal new born infants" without hesitation, and claimed that it could not "rationally be classed as murder", becaıse these children were not yet conscious.26 

Haeckel wanted all the sick and deformed, who may be an obstacle to the so-called evolution of society, not just children, to be eliminated as a requirement of the "laws of evolution." He opposed treatment for the sick, claiming that this obstructed the workings of natural selection. He complained that "Hundreds of thousands of incurables—lunatics, lepers, people with cancer etc—are artificially kept alive in our modern communities…without the slightest profit to themselves or the general body." He further recommended that a commission should be set up to decide the fate of individuals. Upon the decision of the commission the "'redemption from evil' should be accomplished by a dose of some painless and rapid poison."27

This barbarism, upon which Haeckel built his theory, was to be put into practice in Nazi Germany. Shortly after coming to power, the Nazis instituted an official policy of eugenics. The mentally ill, the deformed, the blind from birth, and those with genetic diseases, were gathered up in "sterilization centers." These people were regarded as parasites that spoiled the purity of the German race and its evolutionary progress. Some time after being separated from society, they were eventually killed under special orders from Hitler.

It is a well known fact, pronounced by many historians who have studied the subject, that Ernst Haeckel's ideas and the Darwinist ideology in general, were the ideological basis of Nazism. In his book The Scientific Origins of National Socialism: Social Darwinism in Ernst Haeckel and the German Monist League, the American historian Daniel Gasman presents extensive proof of this. According to Gasman, Haeckel "became one of Germany's major idealists for racism, nationalism and imperialism."28 Haeckel left Nazism an organizational and an ideological legacy. On the one hand he developed the theory of eugenics and racism, and on the other he founded the "Monist League," an atheist association, and this played a major role in the effect the Nazis had on the educated section of society.

Cambridge historian and London Times journalist Ben Macintyre explains the Darwinist thought that Haeckel left as his legacy to the Nazis:

The German embryologist Haeckel and his Monist League told the world, and in particular, Germany, that the whole history of nations is explicable by means of natural selection: Hitler and his twisted theories turned this pseudo-science into politics, attempting to destroy whole races in the name of racial purity and the survival of the fittest...Hitler called his book Mein Kampf, "My Struggle," echoing Haeckel's translation of Darwin's phrase "the struggle for survival."29

This Darwinist influence at the root of Nazism and other fascist ideologies will be examined more closely in other articles.