Darwin considered the evidence from embryology to be by far the strongest single class of facts in favor of his theory.
The evidence was illustrated in a series of drawings by biologist Ernst Haeckel that depicted the growth of embryos from various classes of vertebrates (animals with backbones). The pictures show that the embryos start out looking virtually identical, but as they develop, their appearances diverge until they take the form of their particular class.
To Darwin, similarities in the early embryos indicated that they descended from a common ancestor. As new organs or structures evolved, these features were tacked onto the end of an organisms embryonic development. As a result, we can virtually see the organisms evolutionary history in each embryos development..
Haeckels embryos seem to provide such powerful support for Darwins theory that some version of them can be found in almost every contemporary textbook dealing with evolution.
For example, in the third edition of the college text, Molecular Biology of the Cell, the authors of which include a Nobel laureate and the president of the National Academy of Sciences, states that the embryos of different species "often resemble each other in their early stages and, as they develop, seem sometimes to replay the steps of evolution.
However, Haeckels drawings are wrong. Scientists have known for over a century that vertebrate embryos look very different right from the beginning. For example, although Haeckel depicted the embryos of humans, chickens, frogs, turtles and fish as looking very similar at what he calls the first stage of development, embryologist Adam Sedgwick remarked in 1894 that he could tell the difference between such closely allied species as chickens and ducks at a far earlier stage.
The facts show that the embryos start out looking dissimilar, converge somewhat a few stages later (though not nearly as much as Haeckel depicted) and then diverge again.
Unfortunately, Haeckels depictions are not mere errors. In 1997, a panel of international experts systematically compared Haeckels drawings with actual photographs of embryos. Summing up the panels findings in an interview with Science, the studys principal author said of Haeckels drawings, It looks like its turning out to be one of the most famous fakes in biology.