I personally have nothing against the term "Human Nature". But I think it is easy to reconstruct Buller's meaning here. Our "nature" has clearly evolved; evolution takes place (in part) as a result of variation in a population; evolution of our "nature" is still taking place; hence there is still variation in the "nature" of the human population; hence the whole concept that the species has an essential "Human Nature" is flawed. We are diverse.

I'm not sure I would want to call that kind of word chopping "clueless". But I would point out that the diversity in human nature is the result of the last 150,000 years or so of our evolution, whereas our shared evolutionary history (creating an "essence" of human nature) spans a period roughly 40 times as long.