The roots of phylogeny: how did Haeckel build his trees?

Abstract

Haeckel created much of our current vocabulary in evolutionary biology, such as the term phylogeny, which is currently used to designate trees. Assuming that Haeckel gave the same meaning to this term, one often reproduces Haeckel's trees as the first illustrations of phylogenetic trees. A detailed analysis of Haeckel's own evolutionary vocabulary and theory revealed that Haeckel's trees were genealogical trees and that Haeckel's phylogeny was a morphological concept. However, phylogeny was actually the core of Haeckel's tree reconstruction, and understanding the exact meaning Haeckel gave to phylogeny is crucial to understanding the information Haeckel wanted to convey in his famous trees. Haeckel's phylogeny was a linear series of main morphological stages along the line of descent of a given species. The phylogeny of a single species would provide a trunk around which lateral branches were added as mere ornament; the phylogeny selected for drawing a tree of a given group was considered the most complete line of progress from lower to higher forms of this group, such as the phylogeny of Man for the genealogical tree of Vertebrates. Haeckel's phylogeny was mainly inspired by the idea of the scala naturae, or scale of being. Therefore, Haeckel's genealogical trees, which were only branched on the surface, mainly represented the old idea of scale of being. Even though Haeckel decided to draw genealogical trees after reading On the Origin of Species and was called the German Darwin, he did not draw Darwinian branching diagrams. Although Haeckel always saw Lamarck, Goethe, and Darwin as the three fathers of the theory of evolution, he was mainly influenced by Lamarck and Goethe in his approach to tree reconstruction.