Roux's Entwicklungsmechanih and the Biogenetic Law

Embryology is an historical science only in part. Could that part possibly maintain its popularity against the aggressive supporters of experimental methods and mechanistic outlooks? The experimental method had triumphed in physiology and promised—despite Du Bois-Reymond's ignorabimus ("we will never know")—to reduce organic function to the exact laws of physics and chemistry (Hertwig, 1901). Even Haeckel paid lip service to the ideal of reduction.²⁴

His's attack had come about ten years too early. In its time it was an isolated incident; but by the late 1880s and early 1890s, two of Haeckel's apostate students—Wilhelm Roux and Hans Driesch—were advancing experimental methods in embryology and relegating the biogenetic law to a backshelf of outmoded methods.²⁵ Before the century's end, T. H. Morgan (1899, p. 195) could write: "If I mistake not, there is a tendency at present, that is slowly gaining ground, to give up as unprofitable the interpretation of ... embryological phenomena in terms of speculative phylogeny." This time, proximate causation triumphed and set the fashion for the next half-century, one of the most exciting and fruitful periods in the history of embryology.²⁶

Experimental embryologists rejected all aspects of Haeckel's methodology (see p. 187). They were interested in how the structures of juvenile stages worked; they experimented by disturbing the normal course of development; they studied embryonic stages to discover their proximate causes in previous conditions and to assess their influence upon following ones. In attempting to reduce the complexities of development to laws of physics and chemistry, they focused upon the earliest stages, which recapitulationists usually ignored (patterns of cleavage might yield to mechanical analysis, though the morphogenesis of complex organs seemed intractable). But the greatest clash between the two approaches took place on the battlefield of causality. Experimental embryologists relentlessly asserted that their kind of cause (proximate and efficient) exhausted the legitimate domain of causality. All that had come before them was merely descriptive; they had established the first causal science of embryology. Developmental mechanics [Entwicklungsmechanik] would solve the riddles of ontogeny that had, heretofore, only been recorded in their proper sequence. Thus, Wilhelm Roux began the prolegomenon to his new journal —Archiv für Entwicklungsmechanik—with these words: "Developmental mechanics . . . is the doctrine [Lehre] of the causes of organic forms . . . We may designate as the general goal of developmental mechanics the ascertainment of formative forces or energies" (1894, p. 1). Papers in the Haeckelian tradition were simply ruled