Ernst Mayr (1951):
It is very probable that additional finds will make the delimitation of sapiens against Neanderthal even more difficult. It seems best to follow Dobzhansky's suggestion and to consider the two forms, as well as the ancestral group that seems to combine their characters, as a single species.
Mayr, E. (1950). Taxonomic categories in fossil hominids. In Cold Spring Harbor Symposia on Quantitative Biology (Vol. 15, pp. 109-118). Cold Spring Harbor Laboratory Press.