Connections between ontogeny and phylogeny can be observed in most species. For instance, humans evolved from fish; human embryos pass through a stage with gill-like structures and with webbed fingers. The common ancestor of humans and apes had a tail, and so do human embryos at some stage. Nevertheless, the biogenic law of a strong one-to-one correspondence between ontogeny and phylogeny is rejected by modern biology. Human embryos don't look like fish; they look like fish embryos. A more accurate version of the theory therefore claims that the development stages of a species' embryo resemble the embryonic forms of its evolutionary ancestors. But even this is not always correct: sometimes evolutionary stages are "skipped" in the embryo, and occasionally the order of evolutionary stages is reversed in ontogeny.
The theory can be explained if one assumes that a species changes into another by a sequence of small modifications to its developmental program (which is specified by the genome). Modifications to the developmental program that affect early steps of the program will require modifications in all later steps and are therefore less likely to succeed. Most changes will therefore affect the latest stages of the program and will retain earlier steps. Occasionly however, a modification of an earlier step in the program does succeed and that is when violations of the above rule are observed.
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