Anyhow, Haeckle's theory of the Biogenetic Law is largely debunked.
Well, yes and no. While it's true that species don't recapitulate their entire evolution fetally - an idea stemming from the flawed notion that evolution was a straight-line process from simple organisms to more complex ones - it is now known that Haeckel was looking in the right place, at least. It's known now that the process of evolution of morphology happens primarily in the form of changes to the genetic program of a species' fetal development.
I mean, obviously - making changes to the blueprint of a building before the building is built results in changes to the building. But after the fact, drawing in an extra wall or bathroom onto the blueprint of a building long since finished produces nothing at all.
That said there are many cases where developing embryos do seem to recapitulate aspects of their phylogeny. Whale embryos, for instance, develop and then lose hair and legs. It's not clear why this should be the case unless whales inherit, evolutionarily, the bulk of the fetal development program shared by other mammals.
Edited by crashfrog, : No reason given.