Oh horror. I believed this until right now. Not exactly literally. I never knew Haeckels strong thesis. But from what I had been told (probably as contracted as "ontogeny recapitulates phylogeny") I still assumed the linearity part to be true. And I passed this wrong simplification on to my children (probably like your father did). So now I have some damage control to do...
Indirect quotes from the cited Wikipedia article:
Darwin's view, that early embryonic stages are similar to the same embryonic stage of related species but not to the adult stages of these species, has been confirmed by modern evolutionary developmental biology.
"Embryos do reflect the course of evolution, but that course is far more intricate and quirky than Haeckel claimed. Different parts of the same embryo can even evolve in different directions. As a result, the Biogenetic Law was abandoned, and its fall freed scientists to appreciate the full range of embryonic changes that evolution can produce—an appreciation that has yielded spectacular results in recent years as scientists have discovered some of the specific genes that control development."
What is something you used to believe, preferably something concrete with direct or implied predictions, that you now know was dead wrong. Was your belief rational given what you knew and could know back then, or was it irrational, and why?
Edit: I feel like some of these are getting a bit glib and political. Please try to explain what false assumptions or biases were underlying your beliefs - be introspective - this is LW after all.