Charles Darwin, a British naturalist who took a life-changing trip to the Galapagos Islands on a ship called the H.M.S. Beagle, announces that at the urging of colleagues like Sir Charles Lyell and Dr. Joseph Dalton Hooker, he will finally publish some of the theories that he has been working on for a while. At the center of Darwin’s new ideas is the theory of natural selection—the idea that all species are in competition with each other to exist and that the specimens with the best chances of surviving are “naturally selected.”
Darwin goes on to describe natural selection in more detail. He explains how all individuals within a species have slight variations, and that over millions of years, these variations could be passed down hereditarily. This can then cause the variations to become more pronounced, even giving birth to new species. Whether an organism uses or doesn’t use a body part seems to affect what traits get passed down and survive in a population. Some traits might not have an obvious link to survival; for example, the colorful feathers on some male birds play a vital role in helping them to attract mates, even though the feathers aren’t necessary for the bird to acquire food or avoid predators. (This process is called sexual selection.) To make natural selection easier to understand, Darwin draws frequent comparisons to domestic breeding, showing how what experienced humans do with domestic plants and animals isn’t all that different from what happens in nature when organisms are left on their own to compete in their environment.
Darwin devotes chapters and sections of his book to expand on some of the most interesting or difficult parts of his theory. For example, rudimentary parts of an organism (that is, body parts that aren’t useful), such as the wings on an ostrich, might seem difficult to explain if natural selection leads to adaptations that improve an organism’s chances to survive and pass on its genes. Nevertheless, Darwin shows that because of the principle of economy of growth, natural selection could lead once-useful parts to become rudimentary—since “giving” to one part of an organism usually involves “taking” from another part, rudimentary features allow an organism to adapt in other ways. Similarly, it might seem strange that the offspring of some hybrid crosses between organisms turn out to be infertile. Darwin concedes that the laws behind sterility are complex but argues that, while hybrid sterility is not a result of natural selection, it also does not necessarily contradict the theory.
Darwin often stops in the book to address the concerns of his critics. Darwin insists that species change over time and are therefore not immutable (which means unchanging): some interpret this as a challenge to traditional religious teachings. By Darwin’s own admission, one of the most serious objections to his theories is the eye, which seems like such a complex, highly functioning organ that it could only have been created all once (usually by a Creator, such as God, according to these critics). Darwin, however, provides evidence to show why even the worthiest objections to his theories can nevertheless be answered. In the case of the eye, for example, he shows how many past and present organisms actually have eyes that fall on a spectrum—less complex than the human eye but more complex than a simple optical nerve. Looking at all these examples, it is possible to imagine how even the most complicated eye could have been naturally selected, through a series of small but significant improvements, over a very long period of time. He still allows that issues like the imperfections of the geological record make his job difficult, but he nevertheless promises to go into even greater detail on some of his theories in future books and treatises.
Darwin argues that species don’t arise in multiple places; they more likely originate once and then migrate around the world, developing new adaptations over time. Occasionally, in geographically isolated locations, they form endemic species. Ultimately, Darwin takes this line of thinking to its logical extreme, suggesting that if you go back far enough, all life descends from just a few progenitors or perhaps even just one.
Darwin ends the book with a concise summary of all the issues he covered in the previous chapters. He reiterates that he believes his ideas are not inconsistent with the idea of a Creator and that he believes that evolution simply means that all creatures are on their way towards becoming more beautiful and wonderful.
