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We now know that such adaptations are of comparatively small importance, and cannot be utilized for establishing groups higher than genera in a natural or phylogenetic classification.

It is of importance to know that according to palaeontological investigation, empiric systematizing and phylogenetic classification do not always coincide, as, for instance, in the case of the ammonites.

Upon the Gastræa theory Haeckel based a system of phylogenetic classification which was intended to replace Cuvier's and von Baer's doctrine of Types.