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.
This quotation from Chapter III addresses the way in which variations in a species influence the “perfect” adaptation of an organism to its environment. In the previous two chapters, Darwin explains the different types of variations seen in plants and animals that help scientists distinguish between varieties and species (although Darwin argues that the distinction between the two categories is fundamentally arbitrary). In this quotation, he says that these variations not only produce distinct varieties of species but also create species that are uniquely adapted to their environments. Woodpeckers, mistletoe plants, and parasites have adapted structures (the beaks of woodpeckers, for example) that allow them to maximize their access to nutrients and therefore to survive. The term “co-adaptations” implies that species adapt to one another. For example, the hair or feathers of animals adapt the texture that allows a parasite to cling. Variations, therefore, cannot be random.