For a long time that literally was the dominant philosophy in the west: https://en.wikipedia.org/wiki/Great_chain_of_being
Of course such ideas never explain gun powder or Genghis Khan but nobody ever said racists are smart.
Only because we know better now. It must have seemed self evident at the time and we can forgive Darwin for making that mistake. He was a scientist and I'm sure he'd have accepted the evidence that all the so-called races are embarrassingly alike.
Not a fan of these retrospective moralistic takes.
Wasn't exactly that one of the most revolutionary insights of his theory? That humans evolved and are governed by the same forces of natural selection as other animals? It is amusing that, 166 years after On the Origin of Species, that part is still controversial.
[1] And there are plenty of species that we consider distinct, that can and do interbreed and bear fertile offspring.
