Haeckel was a clever guy, and although he didn't quite hit the nail on the head with ontogeny recapitulating phylogeny, he did recognize the importance of developmental timing in differences among species. Developmental timing is also important within the same species, with small changes during the embryonic level having dramatic effects at the adult level.
The fancy word for a change in developmental timing is heterochrony, which includes any of the following:
i A change in the start or end of a developmental stage i A change in the rate of a process i The loss of the developmental stage
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