Career
In 1900, he became professor at the University of Applied Sciences Stuttgart and in 1909 at Martin Luther University of Halle-Wittenberg. He was president of the Deutsche Zoologische Gesellschaft from 1922. He died unexpectedly from a stroke.
Haecker"s contributions span the fields of ornithology, plankton (Radiolaria) cell biology, developmental physiology, genetics (where he established the subfield of phenogenetics).
He also published a historical treatise on Goethe"s work on morphology.