lightly upon the issue that most of his colleagues had ignored completely: Why do the lower animals stop their development at an intermediate station on the single track leading to man? Serres argued that they must simply contain less of whatever it is that propels development: "Since the formative force, whatever it is, has less energetic impulse [in lower animals] than in higher animals, the organs run through only a part of the transformations that they undergo in superior creatures. From this it follows that they offer to us, in a permanent manner, the organic configurations that are only transitory in the embryo of man and the higher vertebrates" (1830, p. 48).

Recapitulation and the Theory of
Developmental Arrests

We have, thus far, spoken of recapitulation as an almost passive consequence of early nineteenth-century biological philosophies. But was it only a deduction, albeit a colorful one, from prior principles? Were the examples that illustrated it useful only as reflections of these principles, or did recapitulation serve the function of any fruitful scientific hypothesis: did it suggest new ideas and help to generate new data?

Teratology, the study of abnormal development, has always exerted a strange fascination over scientists. Many French and German anatomists, Serres included, had been trained in medicine and had opportunities to receive and dissect seriously deformed fetuses. Oken had spoken of the lower animals, metaphorically to be sure, as so many human abortions. Since the human fetus passes through stages representing lower animals, many abnormalities might be explained as arrests of development. If different parts of the fetus can develop at different rates, then "monstrosities" will arise when certain parts lag behind and retain, at birth, the character of some lower animal. And if, as Serres believed, development is regulated by a formative force of some kind, then a local arrest indicates a local deficiency of force; it might, in principle, be curable. "If the formative force of man or the higher vertebrates is arrested in its impulse, it reproduces the organic arrangements of lower animals . . . These cases of pathologic anatomy are only a prolonged embryogeny" (Serres, 1830, pp. 48–49).

Serres (1860, pp. 534–549) dissected a seriously deformed fetus that lacked a head (Fig. 3). Since clams are the highest acephalous invertebrates, Serres sought other points of resemblance with mollusks in attempting to identify the stage of arrest for this monstrosity. He