The Origin of Species: Chapter 6 Summary & Analysis

Darwin acknowledged that his readers might have objections at this point in the book and stated that he wanted to address them. He identified four main objections: 1. If species all descend through gradual variation, where is the record of all the transitional forms? 2. How could an animal currently living (like the bat) have been formed from another animal with such different habits? 3. Can habits and instincts really be inherited through natural selection? 4. How come when species are crossed the offspring are sterile, but when varieties are crossed, the offspring are still fertile?

On the Absence or Rarity of Transitional Varieties. Darwin addressed the first objection by noting that the geological record is imperfect—perhaps even more so than some of his colleagues believed. More challenging to Darwin was the question of why intermediate forms seem to be so rare among currently existing species.

Darwin observed in his travels that species were generally limited to inhabiting a continuous area. He found that in general, whenever two similar varieties of a species range over large, continuous areas, there is a third variety that exists in a small intermediate zone between them. The small population size means that the varieties in this zone are much rarer and more prone to extinction. This explains why species are relatively well-defined instead of being a chaotic mix of intermediate forms.

On the Origin and Transition of Organic Beings with peculiar Habits and Structure. Darwin noted that some of his opponents questioned how it was possible that a terrestrial carnivore could have descended from an aquatic one. Darwin admitted it was a difficult problem and that he had only collected a few specimens that seemed to show transitional species of this sort. Bats presented a similar problem.

Nevertheless, however, Darwin believed that modern creatures like flying squirrels provided an example of what a transitional species between bats and other mammals might look like. He also noted that many crustaceans and mollusks can breathe underwater but are adapted to live on land.

Organs of extreme Perfection and Complication. Darwin confessed that it seemed at first absurd to him that something as complex as the eye could have arisen through natural selection. Nevertheless, he believed that by following reason, it was possible to imagine the gradual development of the eye over time, with early imperfect versions being subtly improved upon over generations.

Because the imperfections of the geological record make it impossible to look back at a species’ descent in a straight line, Darwin believed that explaining the eye involved looking at different species and genera within a larger group. He found that some species have a very simple organ that could nevertheless be called an eye, with just an optic nerve, some pigment-cells, and some translucent skin around it. Other species have slightly more complex eyes, incorporating pupils but still lacking complex features like lenses.

Ultimately, Darwin observed a wide range of eyes and believed these were evidence that the human eye could have been perfected through gradual adaptations over millions of years.

Modes of Transition. Darwin believed that if there existed an organ so complex that it couldn’t have been formed through a process of slight, successive variations, it would break his whole theory; he also believed there was no solid evidence that any such organ existed. While human organs may seem complex, in many cases they have analogous forms in less complex animals that hint at what transitional organs may have looked like.

Special Difficulties of the Theory of Natural Selection. Darwin addressed some of the most serious criticisms of his theory of natural selection. Neuter insects were a strange case that Darwin promised to explore in the next chapter. The electric organs of some fish also seemed odd, but Darwin argued that the real difficulty here was that so little was known about these organs to begin with, making it difficult to infer how they might have developed gradually. Luminous organs in insects presented a similar issue.

Darwin noted that with electric organs and luminosity, a similar function was performed by organs that might have developed in very different ways. The wings of a bird, a bat, and a butterfly are similar in some ways but also very differently constructed, suggesting a different development process. Darwin summed up this situation by quoting another naturalist who suggested that Nature presented a lot of variety but not much actual novelty. He reiterated his belief in natural selection and how it acted only in small steps, never taking big, sudden leaps.

Organs of little apparent Importance, as affected by Natural Selection. Darwin found difficulty explaining how natural selection was involved with parts and characteristics of creatures that seemed to have little importance. He cautioned, however, that it was not always easy to tell the difference between an unimportant part and one that only appears that way. It’s also possible that today’s unimportant organs once played a very important role in a progenitor species.

Utilitarian Doctrine, how far true: Beauty, how acquired. Some naturalists protested against Darwin by arguing that some beautiful things in nature seem to have been created for the pleasure of man or a Creator. Darwin argued, however, that beauty is subjective and is not necessarily innate or unchangeable. While he admitted that beauty certainly played a role in sexual selection, he believed that other types of beauty may have simply arisen as a result of other forces like symmetry of growth.

Natural selection doesn’t modify one species exclusively for the good of another species. It can lead to features that hurt other species (like the venomous fangs of a snake), but it can’t lead to structures that solely hurt the organism itself with no benefit. While it might seem like the sting of the bee violates this principle (since the bee dies after stinging), Darwin argued that such an adaptation is good for the species as a whole, and that it is still possible to imagine such a mechanism arising from earlier versions that did not cause the insect’s death.

Summary: the Law of Unity of Type and of the Conditions of Existence embraced by the Theory of Natural Selection. Darwin summarized that while the objections to his theories are worth serious consideration, he believed they all had explanations. He repeated his conviction that the best explanation for the similarities among present organisms (“Unity of Type”) is to look back for a common ancestor. He believed that an even more important principle than Unity of Type was Conditions of Existence, by which he referred to the external conditions of an organism’s life that led to the inheritance of variations and adaptations.