The whole premise is a fallacy.
You cannot build upon a fallacy.
History does not rest on premises but on analysis and interpretation of the facts. It's pretty clear that Darwinian or Darwinian-inspired influences were all the rage in Germany, and Weikert marshalls a variety of sources to support for his argument. That some of these negative influences can be traced back to Darwin himself does not impact the truth of the rest of his substantial analysis, but it does cast doubt on his capacities as a moral philosopher.