The concept of the 'organic individual' in Haeckel's writings

Biological individuality was a hotly debated concept in nineteenth-century German biology, both in botany and in zoology. Much discussion centered on a comparison of higher plants with colonial organisms that are subject to polymorphism and exhibit division of labor among their parts. Building on the work of Matthias Jakob Schleiden, Johannes Müller, Rudolf Leuckart, and especially the botanist Alexander Braun, Haeckel in his writings continued to refine his theory of relative individuality. Haeckel recognized three kinds of individuality: physiological, morphological, and genealogical, the latter two hierarchically structured. These distinctions allowed him to embed in his theory of relative (biological) individuality the threefold parallelism of ontogeny, phylogeny, and classification. For Haeckel, this threefold parallelism provided the strongest proof for Darwin's theory of descent with modification.