This lecture series was based on the annotated reading by Ernst Mayr and Frank Sulloway, two major scholars in evolutionary biology and Darwin studies. Their commentary unpacks Darwin’s “one long argument.” During the intorductroy lecture we laid the groundwork for reading On the Origin of Species, not just as a historical document, but as a revolutionary scientific argument at the time. Understanding the structure of Darwin’s five theories, his strategy in presenting them, and the cultural environment in which he worked provides an important perspecitve before diving into the chapters of the Origin itself.
Darwin’s Origin was not just about convincing readers that evolution had occurred. His deeper goal was to replace the supernatural explanation of life’s diversity with a completely naturalistic one. At the time, this was a radical shift. The idea that complex organisms, structures, and behaviors could emerge through gradual, material processes without design or direction fundamentally challenged creationist thought.
The lecture made clear that Darwin saw the mechanism of natural selection as the key to this transformation. His central argument was that variation exists in all populations, and that this variation is subjected to constant environmental pressures. The individuals best adapted to their surroundings are more likely to survive and reproduce, gradually altering the population over time.
Darwin referred to “my theory” in the singular, but in reality his work introduced five distinct ideas:
Evolution as such (Species are not fixed; change occurs over time)
Common descent (All organisms are connected by shared ancestry)
Gradualism (Evolutionary change happens slowly and continuously)
Multiplication of species (Speciation through population divergence)
Natural selection (Mechanism of evolutionary change)
This breakdown, proposed by Mayr, helps clarify why early reactions to Darwin’s book were so mixed. Some accepted evolution and common descent but rejected natural selection or gradualism. It wasn’t until the Modern Synthesis of the 1930s that all five ideas were widely accepted together.
A major theme throughout the lecture was how Darwin structured his argument as a rebuttal to creationist claims. The Origin is full of examples where observed biological facts make more sense under a naturalistic, evolutionary framework than under the idea of special creation. For example:
Biogeography: Oceanic islands like the Galápagos and Cape Verde Islands have completely different faunas despite similar environments. This contradicts creationist assumptions about "design for place" and supports the idea of colonization from nearby continents.
Blind cave animals: These organisms show close evolutionary ties to local surface species, not to other blind species elsewhere. This makes no sense under independent creation.
Vestigial organs: Structures like the wings of flightless birds or rudimentary eyes in cave species serve no function but make perfect sense as evolutionary leftovers.
This lecture emphasized that Darwin used a surprisingly modern approach. He combined inductive observation, hypothesis generation, and testing, much like today’s hypothetico-deductive model.
Though Darwin claimed to follow the Baconian method of pure induction, his actual process involved theory-driven inquiry. He read broadly, collected data, ran experiments, and used his findings to test emerging hypotheses. His work on seed dispersal, insect behavior, and barnacle classification are just a few examples of how hands-on and detail-oriented he was.
The Origin was published in 1859, at a time when very little was known about inheritance or the molecular basis of variation. Yet Darwin’s theory remains close to our modern understanding, a fact that speaks to the strength of his observations and reasoning.
The lecture also traced Darwin’s personal transformation, from a creationist deeply influenced by William Paley’s Natural Theology to an evolutionist skeptical of design. His Beagle voyage and the subsequent analysis of specimens, especially from the Galápagos, were pivotal in shifting his worldview.
The lecture concluded with a discussion of Darwin’s rhetorical strategy. He describes his book as “one long argument” for the theory of evolution by natural selection. Rather than presenting this theory all at once, Darwin builds his case gradually, beginning with artificial selection and variation under domestication, and leading the reader through a sequence of evidence that culminates in a natural explanation for life’s diversity.
By contrasting evolutionary predictions with the shortcomings of creationist explanations, Darwin ensured that each step of the book contributed to this overarching argument.