After 1900, the selective breeding of a few standard animals for research in the life sciences changed the way science was done. Among the pervasive changes was a transformation in scientists' assumptions about relationship between diversity and generality. Examination of the contents of two prominent physiology journals between 1885 and 1900, reveals that scientists used a diverse array of organisms in empirical research. Experimental physiologists gave many reasons for the choice of test animals, some practical and others truly comparative. But, despite strong philosophical differences in the approaches they represented, the view that it was best to incorporate as many species as possible into research on physiological processes was widespread in both periodicals. Authors aimed for generality, but they treated it as a conclusion that would or would not follow from the examination of many species. After 1900, an increasing emphasis on standardization, the growth of the experimental method and the growing industrialization of the life sciences led to a decline in the number of species used in research. In this context, the selective breeding of animals for science facilitated a change in assumptions about the relationship between generality and diversity. As animals were increasingly viewed as things that were assumed to be fundamentally similar, scientific generality became an a priori assumption rather than an empirical conclusion.