How have all those exquisite adaptations of one part of the organisation to another part, and to the conditions of life, and of one distinct organic being to another being, been perfected? We see these beautiful co-adaptations most plainly in the woodpecker and missletoe; and only a little less plainly in the humblest parasite which clings to the hairs of a quadruped or feather of a bird; . . . in short, we see beautiful adaptation everywhere and in every part of the organic world.
Darwin begins the bulk of his argument here, explaining how different species are created. Two concepts dominate this explanation: the struggle for existence and natural selection. Darwin suggests that an organism’s struggle for existence is part of what determines why some species’ characteristics survive and others become extinct. The great number of variations in species have allowed plants and animals to become beautifully adapted to their environments. Darwin provides examples of these adaptations. He mentions the beak of a woodpecker, which allows it to gather insects for food; the structure of a parasite, which allows it to attach to and feed off of another organism; and the ability of a beetle to dive into the water to gather food. These adaptations illustrate how unique characteristics of particular organisms have developed, allowing them to thrive in their specific environments. The most advantageous characteristics are preserved and passed on to offspring. Darwin explains that the presence of these useful adaptations in organisms is the result of natural selection.
Two other concepts, the struggle for life and the limits of population increase, frame the idea of an organism’s drive to exist. Although nature can provide an abundance of food and shelter to its inhabitants, it can also be destructive, causing a struggle for life. Natural disasters, epidemics, and shifts in climate can limit the availability of food and shelter, and animals prey on other plants and animals. Nature inherently disallows the survival of some organisms. Darwin’s principle of the limits of population increase, borrowed from economist Thomas Malthus, is based on the notion that each successive generation of species exponentially increases its population, growing the world population on a constant basis. If each generation continues to reproduce in greater numbers than the one before, and the rate of death remains the same, the earth will eventually run out of room and will be unable to support all of its inhabitants. Therefore, nature limits the number of possible inhabitants of the world. As a result, each individual organism must compete to continue existing, and because there must be a limit on population for every species, one individual organism’s survival inherently threatens the survival of another.
The constant competition for existence compels all organisms and species to strive to outlive others and successfully leave offspring for the survival of the species. Most important to Darwin’s theory is the survival of progeny, because future generations are both dependent on and essential to the perpetuation of advantageous traits and the progress of their race. While much of the competition for existence takes place between members of different species, the most important struggle is between members of the same species. Those individual members who hold advantageous variations that allow them to avoid predators, withstand climate changes, and survive natural disasters have the best chance of surviving. An advantageous variation, combined with successful reproduction, can result in a change in the species, creating a subspecies better equipped to handle its environment. Survival does not occur by chance. Rather, it is the result of advantageous variations.
Finally, Darwin indicates different ways in which the struggle for existence can occur in the natural world. Most cases of survival involve one organism or group possessing an advantage over another one and beating it out. Generally, a species with a larger population has a greater chance of survival than a species with a smaller population, as its larger population makes it less likely to be wiped out by prey and better able to maintain its great numbers through reproduction. In some cases, however, relationships between species govern the chances of survival. Darwin points out that a single tree planted in any area allows further vegetation to grow there and that the fencing off of a section of land to keep cattle away allows seedlings to flourish. In these cases, the survival of a tree or removal of cattle allows the growth of an entirely different species. Struggles for survival are dependent on others, whether those struggles end up being competitive or cooperative in nature.
In this chapter, although Darwin begins to get to the heart of his theory of evolution, he continues to highlight how the work of others helped him form his theory. In particular, he credits economist Thomas Malthus for contributing the theory of a struggle for existence, one of the key concepts driving his own theory. Malthus’s theory provides a rationale for why many species become extinct: In the competition to continue living, the traits that allow species to survive shape the descent of new species, providing the impetus for natural selection. Darwin also uses Malthus’s theory to delve into an entirely different realm of scientific thought, using mathematics and statistics to define the model of population growth. Darwin demonstrates the value of drawing on several scientific fields (in this case, botany, zoology, and mathematics) to construct a scientific theory.
Darwin’s notion of the struggle for existence personifies nature’s contradictions. He reflects on the beautiful and benevolent qualities of nature, which creates many different beings with adaptations that are perfectly constructed for individual organisms’ survival. Darwin also paints a darker picture: Nature can provide abundance for survival, but it can also be destructive. Geography limits population growth, and nature’s changes can destroy life. The search for food also creates inherent destruction, as animals must prey on plants and one another for sustenance. These portrayals of nature as both a benevolent and cruel force can lead to the misinterpretation of natural selection as a clash between forces of good and evil.