In the first case, the darwinists considered he (always definitely a "he") was fully subject to what they imagined were the laws of Nature.
Inside their minds, Nature's laws set groups of individuals, red in tooth and claw, against other groups of individuals, red in tooth and claw, in a total war of attrition within species as to the sole winner in terms of their long term reproductive success.
Germans versus the French, Whites versus Blacks, etc.
(In point of fact Darwin's theory insisted that a new (and "good") mutation usually happened just once, to a single individual, and it was the superior reproductive success of that sole individual and their descendants that oh so very very gradually led to their emergence as a new sub-species group cum strain or race.)
To a much lesser extend, they admitted that their vision of total war to the death happened between species as well : so that, in theory, one day the lion might replace the tigers and become the sole top predator of the Asian jungle.
But if they worried about Russians replacing the Britons and the perilous Yellow Races replacing the Whites, why not also worry that a plant species might replace the human species, species replacing species, as it supposedly did out in Nature ?
(But one could equally argue that when an old niche changed/vanished and the old species that once filled that old niche failed to evolve quickly enough, they disappeared and a new species then arose to fill a new niche, not replaced the old species in the old niche.)
Where are the moral panics that another earthly species will replace Humans ?
Why not worry about an individual mutated Kudzu plant emerging - even more rapidly growing and now poisonous - killing off all other plants by shading them to death, leading to mass starvation up the food chain and reaching even to the most civilized humans ?
"Oh", the darwinists would insist, "Kudzu replacing all the plants might indeed happen in Nature, but Modernity Man is superior to and apart from mere Nature --- our chemists will quickly kill back the Kudzu or even render it highly edible".
In effect, they meant that yes alien Martians, green and scaly but otherwise as smart as us, might replace us - but dumb animals, plants or microbes never could.
"We are not dumb animals, for Heavens Sake", cry these atheists - "we are DEVO".
But if Modernity Man was smart enough to quickly beat back all other species, alien or earthly, why wasn't he equally clever enough to permit a few parents to experience the joy of their severely handicapped child without this swiftly and inevitably leading to certain race suicide ?
Because popular darwinism and modernity was never an internally consistent theory but rather just a grab bag of pseudo rhetoric, designed to sooth the middle class mind without making it work.
Also known as 'retaining your man-is-an-animal cake ---- and yet quickly eating it whenever it suits you' ...