Conclusion

Academicians used the hypothesis of a circulation of sap to search for causal mechanisms. By choosing the Harveian model botanists tried to replace the two principal modes of explaining living things — chemical and mechanical — with a biological one. When the analogy failed, academicians had three choices. They could force a biological explanation by insisting on false structural resemblances. They could fall back on either or both of the traditional modes of explanation. Or they could draw on new physical models that were only half understood. Since the Harveian model itself assumed chemical processes within the physiological and retained a technological model for the heart, recourse to nonbiological explanation was broadly consistent with the model.

Circulatory theory had both substantive and methodological shortcomings. Although a circuit of sap was established, structural resemblances alone between plants and animals could not justify a causal analogue. The effort at methodological equivalence fared no better, despite the experimental ingenuity displayed by Mariotte and La Hire, whose demonstrations of the direction of flow sometimes resemble Harvey's tests. Harvey's crucial experiments were done on living creatures, however, and his theory owed a great debt to his skill at vivisection, but all of Mariotte's and La Hire's dissections were of dead plants. Experiment is necessarily an artificial procedure that may distort its object, and it is least informative when it examines defunct organisms in order to understand physiological processes.^[40]

Given these failures of the analogy, did it have any value at all for seventeenth-century botanists? First, the circulatory analogy had useful consequences despite being a weak form of a partially failed induction. In the absence of a compelling alternative, botanists found a partial analogy better than none at all. Although the causal resemblance failed, a circuit of sap was established. By calling that a circulation, botanists implied that plants enjoyed the digestive and perfecting processes characteristic of animal nutrition. While the circulation of sap could not be subsumed under the laws governing the circulation of blood, the term "circulation" reminded botanists of both the circuit and its function, if not its causal mechanism. Hence, the analogy with the movement of blood supplied a suggestive language to botanists, who retained some of the connotations that the word "circulation" had grown to have for natural philosophers.

Second, because academicians used the experimental and observational form of analogy, their analogy was both positively and negatively useful. It aided "the investigation of structure and the relation of structure and function" and helped reveal some "properties hitherto unnoticed."^[41] It also led academicians to ask new questions and to propose different causes, causes that were testable. Because the extension of the metaphor was more limited structurally than had been foreseen, however, botanical investigation did not affect notions about the physiologies of animals or "plant-animals." Finally, those who did not insist on putative valves in plant vessels learned from the circulatory analogy that there was no organ in plants to make sap rise.

Third, when a model cannot lend its mechanism because of structural disanalogies, the principal value of analogical reasoning must be as an inducement to comparative method. In the case of plants and animals, this may be inevitable, for the two types of organisms enjoy similar functions but dissimilar structures. Analogies used experimentally can draw attention to these problems. But for analogy to work as comparative method, the researcher must not assume the identity of the two things being compared. Therefore, an analogical argument must start by elucidating similarities and differences, as Hesse has pointed out. In this case, the analogy with animals forced botanists to ask how plants accomplish certain functions without having the appropriate organs. By retaining the analogy while admitting structural dissimilarity, academicians moved from analogical to comparative method. That is, they used the analogy to locate specific resemblances and differences; they then tried to explain the differences by comparing the causal mechanisms of the two. In the case of the Academy's study of sap, this transition was incomplete. Some academicians

persisted in pressing the structural comparison by searching for valves. No one seems to have questioned the functional analogy at all, so that the physiology of plant and animal nutrition was assumed rather than tested.

Academicians simultaneously escaped from and succumbed to the dangers of analogy. Although they experimented and acknowledged numerous and crucial dissimilarities, they also assumed fundamental resemblances without examining them. In matters of nutrition, analogy did substitute for experiment. Moreover, when La Hire was driven to find nonexistent valves, he let the model become axiomatic.

The Academy's circulatory analogy could be disproved by experiment and observation. Unlike Harvey's own analogies, which were didactic or were instances of similarity chosen to promote general plausibility, the analogy between sap and blood was falsified by significant dissimilarities. As Canguilhem has pointed out:

A good hypothesis is not always that which leads rapidly to its own confirmation, … It is that which obliges the researcher, by dint of an unforeseen discord between the explanation and the description, either to correct the description or to reconstruct the schema of explanation.… [I]n biology the models which have the chance of being the best are those which halt our latent tendency to identify the organic with its model[.]^[42]

The hypothesis of the circulation of sap had merit in drawing attention to a central problem in botany, namely, how sap rose at all.

It further represents an early attempt to use biology itself as a model for biological explanation. Academicians helped transform botany by finding new explanatory models. But when the biological model failed to supply a causal mechanism, academicians resorted to nonbiological causes, a fruitful reliance that affirmed the interdisciplinary character of scientific explanation. It was not "the impossibility of explaining how the vegetable machine works solely by the laws of motion"^[43] that drove botanists to zoology for inspiration. Rather an incomplete correlation between the zoological model and the vegetable explicandum forced savants back to nonbiological explanation. Seventeenth-century botanists and anatomists found that mechanics, physics, and chemistry were necessary weapons in their explanatory armory. Far from demonstrating a failure of self-image in the biological sciences, the resort to chemistry and physics exemplifies the cross-disciplinary fertilization so important for early modern science, whose practitioners were adept in many fields. Moreover, it reveals a nondogmatic use of analogy. Academicians proved some resemblances but also identified crucial dissimilarities that led them to identify an important scientific problem whose solution lay outside the original analogy. In so doing, they transformed analogical reasoning into comparative method.