"Finally, it may not be a logical deduction, but to my imagination it is far more satisfactory to look at such instincts as the young cuckoo ejecting its foster-brothers, -- ants making slaves, -- the larvae of ichneumonidae feeding within the live bodies of caterpillars, -- not as specifically endowed or created instincts, but as small consequences of one general law leading to the advancement of all organic beings, namely, multiply, vary, let the strongest live and the weakest die."
Now, that can sound like an argument for eugenics to you. What it sounds like to me is an explanation of how some of the more "strange and odious" instincts of animals came into being without falling back on the will of a deity - one that a person could be forgiven for thinking finds delight in creating needless suffering.
It should be noted that "eugenics" is simply nothing more than "selective breeding" applied to people, which is the exact opposite of natural selection. And it should be noted that selective breeding was well-established by Darwin's time - the idea that was only the understanding that nature exerted selection pressure of its own that somehow gave people the idea to treat other people like cattle is fallacious beyond all reasoning.