Darwin's theory is based on the notion of variation. It argues that the numerous traits and adaptations that differentiate species from each other also explain how species evolved over time and gradually diverged. Variations in organisms are apparent both within domesticated species and within species throughout the natural world. Variations in colors, structures, organs, and physical traits differentiate a multitude of species from one another. Heredity is the mechanism that perpetuates variations, Darwin argues, as traits are passed from parents to offspring. What is important about these variations to Darwin, though, is the way they allow species to adapt and survive in the natural world. He gives numerous examples of variations that illustrate the wondrous adaptations that allow species to survive in their natural environments: the beak that allows the woodpecker to gather insects, the wings that allow the bat to fly, the paddles that allow the porpoise to swim, and so on. Darwin hypothesizes that the minor variations we see within a single species-such as variations in size, shape, and color of organisms-are related to the more distinct variations seen across different species. His theory of evolution explains how variations cause the origin of species.