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How does biology explain the living world?

Natural History, May, 1997 by Ernst Mayr

"I wanted to write a life history' of biology that would introduce the reader to the importance and richness of biology as a whole," writes Mayr in his new book.

When biologists try to answer a question about a unique occurrence, such as "Why are there no hummingbirds in the Old World?" or "Where did the species Homo sapiens originate?" they cannot rely on universal laws. Biologists have to study all the known facts relating to the particular problem, infer all sorts of consequences from the reconstructed constellations of factors, and then attempt to construct a scenario to explain the observed facts of this particular case. In other words, they construct a historical narrative.

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Because this approach is so fundamentally different from causal-law explanations, the classical philosophers of science--coming from logic, mathematics, or the physical sciences--considered it inadmissible. However, recent authors have vigorously refuted the narrowness of the classical view and have shown not only that the historical-narrative approach is valid but also that it is perhaps the only scientifically and philosophically valid approach in the explanation of unique occurrences.

Of course, proving categorically that a historical narrative is "true" is never possible. The more complex a system is with which a given science works, the more interactions there are within the system--and these interactions very often cannot be determined by observation but can only be inferred. The nature of such inference is likely to depend on the background and the previous experience of the interpreter; therefore, not surprisingly, controversies over the "best" explanation frequently occur. Yet every narrative is open to falsification and can be tested again and again.

For instance, the demise of the dinosaurs was once attributed to the occurrence of a devastating disease to which they were particularly vulnerable, or to a drastic change of climate caused by geological events Neither assumption was supported by credible evidence, however, and both ran into other difficulties. Yet in 1980 when the asteroid theory was proposed by Walter Alvarez--and, particularly, after the presumed impact crater was discovered in Yucatan--all previous theories were abandoned, since the new facts fit the scenario so well.

Among the sciences in which historical narratives play an important role are cosmogony (the study of the origin of the universe), geology, paleontology, phylogeny, biogeography, and other parts of evolutionary biology. All these fields are characterized by unique phenomena. Every living species is unique and so is, genetically speaking, every individual. But uniqueness is not limited to the world of life. Each of the nine planets of the solar system is unique. On earth, every river system and every mountain range has unique characteristics.

Unique phenomena have long frustrated the philosopher. David Hume noted that "science cannot say anything scientifically about the cause of any genuinely singular phenomenon." He was correct if he had in mind that unique events cannot be fully explained by causal laws. However, if we enlarge the methodology of science to include historical narratives, we can often explain unique events rather satisfactorily, and sometimes even make testable predictions.

The reason historical narratives have explanatory value is that earlier events usually make a causal contribution to later events. For instance, the extinction of the dinosaurs at the end of the Cretaceous vacated a large number of ecological niches and thus set the stage for the spectacular radiation of the mammals during the Paleocene and Eocene. The most important objective of a historical narrative is to discover causal factors that contributed to later events in a historical sequence.

How does biology explain the living world?

Natural History, May, 1997 by Ernst Mayr

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