Penn et al. do not demonstrate Darwin made a mistake, because they largely ignore the semantics underlying the meanings of “degree” and “kind.” An analysis based on the work of Mortimer Adler shows such terminology conflates at least three different meanings of “kind,” only one of which challenges Darwin – and one which the authors almost certainly would reject.
We must also admit that there is a much wider interval in mental power between one of the lowest fishes, as a lamprey or lancelet, and one of the higher apes, than between an ape and man; yet this immense interval is filled with numberless gradations.;>
— Charles Darwin (1871, p. 35)
The distance between man and ape is greater than the distance between ape and ameba.;>
— William Gaylin (1990, p. 8)