Engels uses both the neutral term "negroes" and the derogatory "nigger"

 Note that Engels uses both the neutral term "negroes" and the derogatory "nigger". So he clearly knew what the different implications of the two terms were. "Nigger" was not as verboten in the 19th century as it is now but it was still derogatory -- and it is presumably because of that aspect of the word that both Marx and Engels used what is after all an English word in their German writings.

To undersrtand what Engels was talking about, one needs to realize that both Marx and Engels were Lamarckians -- they believed that acquired characteristics could be inherited. That fact is no doubt part of the reason why Stalin so heavily sponsored the ideas of the Lamarckian Trofim Lysenko right into the 20th century -- long after Lamarckian theories had been generally discredited in the West. And the particular strand of Lamarckian thinking that appealed most strongly to both Marx and Engels was that the type of soil and landscape in which a nation grew up could influence their national character. Just what the relationship between geology and national characteristics was, however, they did not fully agree. The following commentary on the matter may also be helpful:

To cite one final anecdote, the scholarly literature frequently cites Marx's great enthusiasm (until the more scientifically savvy Engels set him straight) for a curious book, published in 1865 by the now (and deservedly) unknown French explorer and ethnologist Pierre Tremaux, Origine et transformations de l'homme et des autres etres (Origin and transformation of man and other beings). Marx professed ardent admiration for this work, proclaiming it "einen Fortschritt uber Darwin" (an advance over Darwin). The more sober Engels bought the book at Marx's urging, but then dampened his friend's ardor by writing: "I have arrived at the conclusion that there is nothing to his theory if for no other reason than because he neither understands geology nor is capable of the most ordinary literary historical criticism."

I had long been curious about Tremaux and sought a copy of his book for many years. I finally purchased one a few years ago--and I must say that I have never read a more absurd or more poorly documented thesis. Basically, Tremaux argues that the nature of the soil determines national characteristics and that higher civilizations tend to arise on more complex soils formed in later geological periods. If Marx really believed that such unsupported nonsense could exceed the Origin of Species in importance, then he could not have properly understood or appreciated the power of Darwin's facts and ideas.

https://marxwords.blogspot.com/