"I established the opposite view, that this history of the embryo (ontogeny) must be completed by a second, equally valuable, and closely connected branch of thought - the history of race (phylogeny). Both of these branches of evolutionary science, are, in my opinion, in the closest causal connection; this arises from the reciprocal action of the laws of heredity and adaptation... 'ontogenesis is a brief and rapid recapitulation of phylogenesis, determined by the physiological functions of heredity (generation) and adaptation (maintenance).'"
Haeckel, E. 1899. Riddle of the Universe at the Close of the Nineteenth Century.
The gill hypothesis tends to uphold Haeckel. For years scientists and their critics have denied that it is true. However, the creationists are overly critical of Haeckel and do not understand the nuance we see in developing embryos, that does present some evidence for evolution.
Haeckel was a great scientist and a genius.