Recapitulation theory
Today, scientists think that Haeckel overstated the case. For example, the "gills" of a developing human do not work (though Haekel never said they did). Furthermore, as better observations were made, it was discovered that developing babies don't look exactly like the pictures Haeckel drew. He may have exaggerated what he saw in order to support his point better. Also, Haekel's discussion was entirely about morphology: he did not discuss the evolution and development of animal behaviour.