The concept of similarity plays a crucial role in biology, especially in natural history. Despite its apparent familiarity it has been subject again and again to reinterpretations — it may even be stated that the main streams of theoretical thinking in the life sciences are reflected and condensed in its ever changing meaning. The changing content of the concept is analyzed from Linnaean systematics through classical morphology and comparative anatomy to Darwinian evolutionary thinking. It appears that the meaning of similarity (...) is inseparable from the function of the concept in theory, viz. its operational determination. In addition a remarkable correspondence has to be recognized between the notion of similarity and the conceptualization of an organism as the basic unit of life. (shrink)