Against this background, we turn to Darwin himself. We first look at the selection analogy in his theorising before writing the Origin. Darwin arrived at his theory of natural selection before contemplating such an analogy. We cannot, then, understand the analogy as what led him to the theory. Its role was to support a theory already arrived at. The evolutionary process takes place over millions of years at an imperceptibly slow pace, and so is inaccessible to direct observation. However, here and now we can observe the selective human breeding of domestic animals and cultivated plants. Darwin can then use an argument by analogy to give his theory indirect empirical support. The struggle for existence in the wild and the human breeders are not intrinsically similar agencies, but are relationally comparable in having the same kind of causal relation to the animals and plants that they are acting on with effects similar in kind but not in degree.