Krause (1909) provided detailed anatomical description of the human skeleton and comparative analysis of fossil forms including fossils from Neanderthal, Spy and Krapina, each of which he assigned to separate species in accord with his assertion that they each represented independent lineages rather than subspecies of Homo primigenius.
Krause (1909) discussed his opinion that the Trinil material is a gibbon, Hylobates gigas. Specifically, he wrote (p. 176),
“Die gemeinschaftlichen Vorfahren des Menschen und Affen, die zuweilen als kletternde Primaten bezeichnet werden, liegen im Eocan begraben. Pithecanthropus hingegen ist ein grosser Affe, Hylobates gigas, der mit der direkten Vorfahrenreihe des Menschen nichts zu tun hat, selbst wenn er sekundar das aufrechte Gehen gelernt hatte.”
The section above loosely translates to,
The common ancestors of humans and apes, sometimes referred to as climbing primates, are buried in the Eocan. Pithecanthropus, on the other hand, is a great ape, Hylobates gigas, which has nothing to do with the direct line of human ancestors, even if it had learned to walk upright as a secondary step.
Campbell (1965) noted that Hylobates gigas based as it is on Trinil 2, is an available though objectively invalid junior synonym to Pithecanthropus erectus.