The Origin of Species by Charles Darwin is a landmark in the history of Biology. It laid down the foundations for the modern theory of evolution and influenced several areas of the Natural History as well as other fields of inquiry. The Origin of Species brings in the theory according to which Natural Selection has been the most important means, although not the only one, of modification and production of new species in Nature. The novelty of Darwin's way of arguing in exposing and defending his theory can be seen in the methods, and explanatory patterns and strategies he creates and makes use of in the Origin of Species. These topics bear on those that have been fundamental to the Philosophy and History of Science, like the ones related to the role of theory and method, the ways of generating and applying concepts, and the patterns of explanaton in science. These questions, in turn, lead us te, think about the concept that is in the background of all discussions on science — the concept of scientific rationality. Usually, it is understood in terms of a cognitive faculty, an attitude, or a value. This way of understanding rattonality receives a richer treatment in Darwin's work, pointing to a network of multiple and interactive meanings.