self: the bud from which fingers differentiate is large when it first develops in the fetus. In normal ontogeny part of it disappears before the fingers differentiate. If the bud is arrested at its original size, too much material remains when the fingers develop and too many fingers appear.

The theory of developmental arrests was both successful and influential (see work of Etienne's son Isidore Geoffrey Saint-Hilaire, 1833); it added much prestige to the concept of recapitulation. Even von Baer, recapitulation's nemesis, had to praise this application: "It [recapitulation] won more influence, as it proved itself fruitful; a series of malformations could be understood when they were considered as the consequence of a partial arrest of development at earlier structural stages" (1828, p. 200). Two things were incontestable: (1) many malformations are arrested embryonic states; (2) this conclusion had been reached with the aid of recapitulation. But, von Baer noted, a third statement, crucial to biological theory, does not necessarily follow—that arrested embryonic stages are comparable to permanent conditions of lower animals (p. 232). Von Baer set out to demolish this third proposition.

Von Baer's Critique of Recapitulation

The Direction of Development and
Classification of Animals

Ernst Haeckel's writings are sparing of praise and generous in skilled rhetoric of withering intensity against opponents. Yet he called von Baer's Entwickelungsgeschichte "the most significant work in the entire ontogenetic literature" (1866, 2:14). And while the aged von Baer was attacking Darwin from his outposts in St. Petersburg and Dorpat, Huxley was referring to him as Darwin's equal (Oppenheimer, 1959).

Karl Ernst von Baer (1792–1876) was a paragon of nineteenth- century science (Raikov, 1968). After studying with Burdach in Dorpat and with Dollinger in Wiirzburg, he received a professorship at Konigsberg in 1819. There he published the first part of his Entwickelungsgeschichte der Thiere in 1828 and reported his discovery of the mammalian ovum in 1827. In 1834 he gave up embryology and moved to St. Petersburg. This sudden decision recalls Rossini's abandonment of opera at the height of his fame and may have had a similar cause: nervous breakdown and the threat of ill health. In Russia, von Baer led expeditions to Novaya Zemlya and the Caspian Sea, founded Russian anthropology, made notable advances in ecology, established the law relating erosion of river banks to the earth's rota-