On the Origin of Species

Darwin makes his first sustained attempt to directly respond to many of the challenges to natural selection theory. He names four main objections to the theory and deals with the first two in this chapter, the third in Chapter 7, and the fourth in Chapter 8.

The first potential objection is the question of why large numbers of gradient forms don’t exist between distinct species blended in blustering confusion. If natural selection is a true theory, this objection holds, “innumerable transitional forms” (671) should be everywhere. The second objection insinuates that the degree of modification requisite to transform one species into something totally different (with different structures and habits) is too bizarre to take seriously.

Darwin responds to the first objection by employing many of the laws of variation and fundamentals of natural selection that he developed in the preceding chapters. Parent species and the gradations of transitional species/varieties preceding the current species are generally replaced by the ancestor. They’re outcompeted in the struggle for life, which is the main cause of natural selection. The success of one species precludes the existence, or at least a large population of, another: “Consequently, evidence of their former existence could be found only amongst fossil remains” (677). Darwin shows in Chapter 9, however, why the fossil record is so flawed.

The next objection concerns the “origin and transitions of organic beings with peculiar habits and structure” (677) and “organs of extreme perfection and complication” (683). Darwin handles these cases similarly. He provides many examples of animals with peculiar habits, like a woodpecker that never climbs a tree, and explains how these animals adjusted to their environment via natural selection. Just because a habit is peculiar, Darwin demonstrates, doesn’t mean it’s not explainable. Proponents of independent creation theory often invoke the eye to defend it, claiming that the eye is too perfect and complicated to result from blind natural selection. Darwin argues against this by noting that the time scale of natural selection makes the development of the eye possible:

Let this process go on for millions on millions of years; and during each year on millions of individuals of many kinds; and may we not believe that a living optical instrument might thus be formed as superior to one of glass, as the works of the Creator are to those of man? (685)

Darwin advises caution when rejecting the view that transitional forms could eventually result in an amazing organ. Conversely, less relevant organs fall into disuse and diminish over time.

In summarizing this chapter, Darwin again urges caution in challenging the capabilities of transitional development. Natural selection proceeds via competition among the inhabitants of an area, so organisms tend to develop only to the degree that is useful to them in their environment. He notes two great laws for organic beings: “Unity of Type” and “Conditions of Existence” (698). The latter is the higher law, which dictates the direction of natural selection. Types vary as variation and the conditions of existence allow.

The third potential objection to natural selection theory asks how it could possibly account for the acquisition of instincts. In this chapter, he sets out to show how. Instincts are actions that animals perform that would require learning experiences for other animals or humans. Instinctual action often occurs at an early age and don’t require that the animal have any cognizance of why it performs the act. Darwin doesn’t entirely dismiss the idea that acquired habits can become inherited, but he doesn’t think they compare in importance or degree to “accidental variations of instincts” (701). These “accidental” variations, Darwin holds, have the same kinds of unknown causes as sudden modification in physical structure. Thus, instincts are just as explainable via natural selection as any other change that develops through descent with modification.

Darwin discusses how some animals take advantage of the instincts of others, even though all animals produce instincts for their own benefit. He notes how the domesticated instincts of dogs are subject to greater variability than naturally occurring instincts because the conditions of their life are less fixed, and the instinct develops over an “incomparably” shorter time. In addition, he notes that natural instincts can disappear in domestic contexts.

To explain how instincts work and develop via the “wonderful” processes of natural selection, Darwin focuses on three case studies: the cuckoo bird that lays eggs in a competitor’s nest, the ants that make slaves out of competitor ant species, and honeybees, which have perfected the art of honeycomb construction. In each case, Darwin illustrates how slow, gradual changes in instinct can account for bizarre, intricate, and unexpected instincts—like those that make bees perfectly efficient architects and those that make ants vicious slave masters. In addition, he examines at length the problem of naturally selected sterility among a subset of the population in certain ant species.

In concluding the chapter, Darwin uses his explanation of these amazing facts of natural instinct to argue once more against independent creation theory:

Finally, it may not be a logical deduction, but to my imagination it is far more satisfactory to look at such instincts as the young cuckoo ejecting its foster-brothers, ants making slaves, the larvae of Ichneumonidae feeding within the live bodies of caterpillars, not as specially endowed or created instincts, but as small consequences of one general law, leading to the advancement of all organic beings, namely, multiply, vary, let the strongest live and the weakest die (727).

Darwin thus holds that his theory better accounts for these facts than the competing theory, which must claim that these creatures are divinely endowed with unseemly instinctual manners of behavior. In this passage, Darwin conveniently introduces the example of larvae feeding on live bodies and neglects to mention honeybees and their combs.