Passage
Classifications of the tube-building marine invertebrates were long organised around tube architecture: mineral composition, the spacing of growth rings, the shape of the aperture. The merit was practical. Tubes persist on collected rock, while the soft animals inside them are frequently torn in collection, so anyone with a hand lens could apply the scheme. When comparative study of larval stages became routine, however, the resulting groupings cut across the older ones: animals whose tubes were nearly indistinguishable proved to pass through markedly different sequences of larval segmentation.
The dispute that followed is often narrated as a contest the larval evidence won. That misdescribes it. Very few participants doubted the larval observations; what they disagreed about was the standing those observations should have. Defenders of the older scheme argued that a classification is an instrument for retrieval and prediction, an office characters visible only in a laboratory, for a few days of an animal’s life, could not perform. Their opponents replied that tube architecture responds to the surface an animal settles on and to the surrounding water chemistry, so similarity of tube is evidence of similar circumstances rather than of common ancestry: a catalogue of habitats wearing the costume of a genealogy.
What ended the argument was not a decisive observation but a division of labour. The formal classification was reconstituted on developmental and molecular grounds, while tube architecture was retained, and explicitly demoted, as a field key: a device for reaching a probable name quickly, to be checked against the formal scheme when the name had to bear weight. Each criterion was thereby handed the question it answers well.
The settlement does not show that the older workers had misread their material; their groupings remain about as serviceable at their intended job as they ever were. Nor does it show that developmental characters are inherently more fundamental — that conclusion holds only so long as such characters are not themselves acquired repeatedly by animals living in similar conditions, which for at least one larval feature in this group remains an open possibility. It shows something narrower: a single classification had been asked to do two jobs, and the appearance of contradiction in the evidence was in part an artefact of that demand.
