For Charles Darwin, comparative embryology was important evidence in support of the evolution of descendants from a common ancestor. The biologist Ernst Haeckel was the German advocate of Darwinism. He developed the theory of an earlier idea that ‘ontogeny (embryonic development) recapitulates phylogeny (evolutionary history)' … his so-called biogenetic law that dominated evolutionary thinking in the late 19th century. Walter Garstang at Leeds University highlighted the problems of the biogenetic law in 1921. He proposed embryonic development represents the stages of ancestors to whatever point it was in ontogeny when divergence into new forms occurred. In effect 'ontogeny does not recapitulate phylogeny - it drives it'. Within historical context, our aim will be to add to our understanding of evolution through natural selection by exploring a definition of evolution as a modification through time of ontogeny as the result of changes in gene expression and regulation.