Beginning chapter 2, "Evolutionary Progress as the Highest Good", Weikart writes p. 43:
In 1904 one of the leading German Darwinian biologists, Arnold Dodel, proclaimed, "The new world view actually rests on the theory of evolution. On it we have to construct a new ethics ... All values will be revalued." Though proponents of of evolutionary ethics did not always agree on the specifics of the new morality, they all agree that present moral norma had to be examined anew in light of evolutionary theory. Some traditional moral norms might be valid, but others must be revised or completely overthrown. Their moral relativism implied that some moral values might have been valid in the past, but may no longer apply under modern conditions.
The moral relativism mentioned here then means an update of moral norms to current conditions, and as such actually a kind of absolutism: there is no choice.
According to Weikart, the idea of an evolutionary ethics emerged. What was good was what furthered evolution, and what was bad was what hindered evolution. This is discussed by Weikart over the next pages, centered around Friedrich Nietzsche. One thing that could have been relevant for Weikart to mention here is that for Nietzsche, fitness was a question of interpretation; the fit was the one who could interpret his vices as virtues and defeats as victories.
Weikart's main point is, of course, that evolutionary ethics is opposed to Christian ethics. Thus, when discussing Wilhelm Schallmayer, Weikart writes p. 51:
Schallmayer attacked Christian morality as an impediment to evolutionary progress. He asserted that "the views of Christianity, insofar as they are at all influential, do not have the tendency to improve selection, either consciously or unconsciously, but rather - naturally - unconsciously - has the opposite tendency." He deemed some of the ideals of Christian morality, such as humility and despising earthly goods, harmful in the struggle for existence and thus irreconcilable with true morality. How then, did Christian morality become so prevalent, if it conferred such disadvantages to its bearers? Schallmayer suggested that some aspects of religious ethics can be advantageous in the struggle for existence. However, those aspects of Christianity most disadvantageous, such as loving one's enemy, survived only because they remained a dead letter, never being put into practice.
Implicit in Weikart's discussion here is that Christian ethics was prevalent. Almost 400 years earlier Martin Luther wrote that less than one out of thousand was a Christian, and making Christian ethics law, as the Anabaptists wanted, would therefore only let loose wild animals (= criminals). For Luther the state of sin was a condition imposed by God, and a strong state with unconstrained power to punish criminals was needed in order to enable peaceful people to live in safety. Thomas Hobbes saw things in much the same way, except he suggested civil and ecclesiastical powers combined in the Sovereign.
In short, here as elsewhere, Weikart is too superficial in his analysis.
Chapter 3, "Organizing Evolutionary Ethics", deals, as the title indicates, with the various organizations with an evolutionary program. One of these was the Monist League, founded by Haeckel. As Weikart writes, the Monist League was opposed to the Nazis and was banned by them. Weikart writes p. 70:
The Nazi suppression of the Monist League was not a function of a fundamental change in the Monist League's orientation during the Weimar period, as [Daniel] Gasman has argued, but rather reflected significant differences between Haeckel and Hitler. Haeckel and the Monist League promoted many social reforms that were anathema to Hitler, such as homosexual rights, feminism, and pacifism. Gasman's Haeckel-to-Hitler thesis ultimately failed, in part because he ignored the many areas of sharp disagreement between Haeckel and Hitler. However, while acknowledging the many differences, we should not ignore the many features of Monist ideology that featured prominently in Hitler's worldview such as eugenics, euthanasia, and social Darwinist racism.
Daniel Gasman, author of The Scientific Origins of National Socialism: Social darwinism in Ernst Haeckel and the German Monist League and Haeckel's Monism and the Birth of Fascist Ideology, writes in chapter 7, "Monism and National Socialism", of the former book:
If one surveys the origins of the Volkish movement in Germany during the three or four decades prior to the First World War it is apparent that Haeckel played an influential, significant, indeed a decisive role in its genesis and subsequent development. An impressive number of the most influential Volkish writers, propagandists, and spokesmen were influenced by or involved in some way with either Haeckel or his Monist followers. In the development of racism, racial eugenics, Germanic Christianity, nature worship, and anti-Semitism, Haeckel and the Monists were an important source and a major inspiration for many of the diverse streams of thought which came together later on under the banner of National Socialism.
It is beyond me to here arbitrate between Weikart and Gasman. However, the Monists were rationalist freethinkers, hardly the kind of people to support Nazism. In return, this would of course not have prevented others from adopting Monist ideas to their own purposes. The question would be, to what extent, if at all, Monists had taken active part in the Volkish ("Popular") movement. The German nationalism leading up to and during the Second Reich played its part in Monism, but the Monists weren't the only players on the field.
In return, because Weikart is specifically tracing Darwinism, he misses the Volkish movement and its Germanic ("Aryan") nationalism, a much more directly political concept. Haeckel's influence might mainly have been channeled though Rudolf steiner's mixture of Monism and Theosophy, the latter adding the mysticism that was so disagreeable to the rationalist Monists.