Marx was talking about Ferdinand Lassalle, another German philosopher and a social democrat. He probably meant that, since Lassalle’s hair was somewhat similar to certain hairstyles common among Africans, Lassalle must be of African descent.
Also it’s quite amusing that Darwin is himself portrayed on the coin in a manner that highlights visual human-ape similarities, since I’ve heard the claim that he considered Africans and certain other dark-skinned people like Australian Aboriginals to be closer to apes than Europeans were.
“No one supposes that one of the lower animals reflects whence he comes or whither he goes, - what is death or what is life, and so forth … How little can the hard-worked wife of a degraded Australian savage, who uses hardly any abstract words and cannot count above four, exert her self-consciousness, or reflect on the nature of her own existence” (The Descent of Man, page 62)
“From the remotest times successful tribes have supplanted other tribes … At the present day civilised nations are everywhere supplanting barbarous nations” (160)
“…talent and virtue were features to be identified solely with Europeans” (243)
