two major interpretations for the significance of embryonic stages. Each had been formulated under creationist tenets, but each could be easily restructured in evolutionary guise. These were, of course, von Baer's principle that development proceeds inexorably from the general to the special and the recapitulationist claim that embryonic stages represent adult forms of "lower" creatures. Both were quickly given their evolutionary meaning: Darwin accepted von Baer's principle but stood the original explanation on its head. F. Miiller, Haeckel, Cope, and Hyatt independently recognized the irresistible promise of recapitulation as a key to the reconstruction of phylogeny.

Do old practices, like good bureaucrats, survive revolutions by enveloping their unaltered core of basic procedure in the appropriate window-dressing of a new theory? Did Darwin and Haeckel merely place old wine in new bottles by refurbishing some terminology? Or did evolutionary theory, in this case, prescribe a new way of proceeding? I shall argue that evolutionary theory transformed the workaday habits of comparative embryologists by posing problems and providing insights that earlier explanations for the same principles had not supplied.

Darwin and the Evolution of von Baer's Laws

On September 10, 1860, Darwin wrote to Asa Gray: "Embryology is to me by far the strongest single class of facts in favor of change of forms." It is often assumed that Darwin had recapitulation in mind,* but Fritz Miiller's first evolutionary interpretation of recapitulation did not appear until 1864, and Darwin was not quoting the Naturphilosophen. In fact, Darwin had accepted the observations of von Baer—a flat denial of recapitulation and its obvious evolutionary meaning. Referring to von Baer, he wrote in his autobiography:

*This common assumption has two rather different bases. First, many authors simply don't read Darwin carefully and assume that his strong invocation of embryology must be based upon recapitulation; the phrases "evolutionary embryology of the nineteenth century" and "biogenetic law" are linked so closely that many authors simply do not recognize other evolutionary readings of embryology. This is an outright error and need detain us no longer. Second, many secondary sources (but no primary participants in the nineteenth century) have extended the word "recapitulation" beyond its original definition as the repetition of adult stages in ontogeny to encompass any belief that phyletic information resides in ontogeny—a proposition that can scarcely be denied by an evolutionist. This has produced the lamentable confusion that I document in the introduction. Lovejoy, for example, uses the phrase "Darwinian theory of recapitulation" in a title, though he recognizes that Darwin's views are von Baer's transformed. He contends that the laws of von Baer are a "denial, not of recapitulation itself, but simply of recapitulation of adult forms"(1959, p. 443).