and positioning (heterotopy) within the body of aspects of embryonic development would change the shape of a descendant's body compared to an ancestor's, were first formulated by Haeckel in the 1870s. These elements of his thinking about development have thus survived, whereas his theory of recapitulation has not.

The Haeckelian form of recapitulation theory is considered defunct. Embryos do undergo a period or phylotypic stage where their morphology is strongly