In addition to the idea of evolution by natural selection, On the Origin of Species illustrated a somewhat different way of looking at biology and a different philosophy of science from that familiar to Darwin's contemporaries (Mayr 1964: xviii).
A New Conception of Biology. For Darwin, transmutation of species was a natural phenomenon: it neither required a guiding hand nor resulted in a predetermined goal. Species changed as a result of the need to adapt to immediate environmental circumstances. Because the geology of the planet, and thus environmental circumstances, changed over time, there could not be an ultimate goal toward which creation was heading. It was not possible to predict future changes in living organisms. Darwin's view of science restricted scientific explanations to natural causes. In this he was preceded and influenced by changes that had taken place during the previous one hundred years or so in the field of geology (Gillespie 1979: 11).
In the late 1700s, the Scottish geologist James Hutton proposed a view that became known as uniformitarianism: that Earth was ancient, and its surface could be explained by processes we see taking place today—sedimentation, erosion, faulting, flooding, and the like. There was no need to invoke the direct hand of God to explain the building up of mountains, the presence of seas modern or ancient, or the accumulation of layers of strata. Geology could be understood through natural processes. Darwin's mentor and friend Charles Lyell promoted uniformitarianism in the 1830s and beyond, and the view came to predominate—though not without opposition.
Uniformitarian geologists eventually won the day, but biologists lagged behind; a seminal uniformitarian text by the Scottish scientist John Playfair, Illustrations of the Huttonian Theory, was published in 1802—the same year that William Paley published his argument for design, Natural Theology. But the seeds for a naturalistic foundation for biology had been planted: geology, after all, has consequences for biology, as fossils partly define the geological column. Different strata are regularly marked by the disappearance of some life-forms and the appearance of new ones (even if they are similar to previous ones). How can these be explained? Creationist geologists required that God re-create life-forms after every catastrophic geological change. Darwin viewed the appearance of new species in a stratum as the result of evolutionary change, of descent with modification from earlier ancestors. His mechanism of natural selection likewise reinforced the conclusion that the fossil record and current diversity of life could be explained without recourse to divine intervention. Darwin's bold naturalism applied to biology proved difficult for many critics to take. Many scientists and theologians objected to Darwin's removal of the need for divine intervention in the biological sciences—much as critics of uniformitarian geology had protested a century before.
Similarly, because there was so much evidence that species had indeed changed through time, and because Darwin's and other scientists' studies of both wild and domesticated animals and plants had demonstrated great variation of form within species, equally untenable were typological species concepts in which species were conceived of as reflections of a Platonic eidos (or idea). Darwin practiced what the modern biologist Ernst Mayr (1964) calls population thinking, in which the object of study of biology is actual individual-to-individual variations rather than an abstract concept of an ideal form.
Perhaps because Darwin was fundamentally a naturalist with broad knowledge of living plants and animals, he was able to conceive of species as having almost unlimited variation, which allowed him to speculate about variation as a source of gradual adaptation and eventual transmutation.
A New Conception of Science. The expectation of scientists in the mid-nineteenth century was that the goal of science was the accumulation of certain knowledge. A successful scientific explanation resulted in positive finality. Anything less than certitude was deficient (Moore 1979: 194).
According to this inductivist approach, the scientist who properly performs his or her craft is one who patiently collects facts, assembles them in a logical and orderly fashion, and lets explanations arise out of this network of ideas. "The outcome of repeated inductions would be a series of propositions, decreasing in number, increasing in generality, and culminating in 'those laws and determinations of absolute actuality' which can be known to be certainly true" (Moore 1979: 194; internal quote from Losee 1972: 164-167). A scientific explanation was considered to have been proved when it accounted for all the facts and thus was a complete and certain law of nature.
Of course, such an ideal is hardly ever obtainable. It is the nature of science that new discoveries cause us to rethink our conclusions and rework our explanations. Today no one thinks that there is ever final certainty to a scientific explanation, but in the late eighteenth century, such a view was common—though not universal—among scientists and other educated individuals. This was not Darwin's approach, however.
Darwin recognized that the world is not static and, with his abundant knowledge of natural history, knew that variability characterizing natural phenomena would make the certainty sought by the strict inductivist approach highly improbable. How could one account for all the facts if new facts were continually being generated? "The lesson was plain: induction, no matter how rigorous, could never rule out the possibility of alternative explanations" (Moore 1979: 196).
Darwin's approach to science indeed was to collect facts (and there is an abundance of them in Origin—Darwin was a skilled natural historian and experimenter), but to collect them with a hypothesis or tentative explanation in mind. He used those hypotheses that were not factually disproved to generate additional hypotheses, which he then tested against the facts. He thereby established a network of inferences. Darwin was careful to state how his hypotheses and generalizations could be tested by listing what sort of observations would have to be made to disprove his views—but he also firmly asserted that, until that time, his explanations were the best available.
Rather than the more familiar approach of presenting his views when they were proved or certain, Darwin's approach was to present a coherent set of supported inferences, arguing that the lack of counterevidence gave them the highest probability of being an accurate or true explanation. The probabilistic approach to science, reflecting a dynamic universe, was a sharp contrast to the older approach of many of Darwin's contemporaries, many of whom viewed the universe as specially designed and largely static. According to Moore (1979), Darwin's approach to science itself was one of the major reasons that the concept of evolution by natural selection presented in On the Origin of Species was rejected. Darwin's great work was denounced as speculative, probabilistic, unsupported, and far from proven. Yet Darwin's way of doing science— probabilities and all—is much more familiar to us in the twenty-first century than is that of his contemporaries.
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