Most biologists, particularly Australian biologists, are aware that the initial description and attempts to classify the echidna and platypus were surrounded by controversy. Fewer are aware of the important roles played by two eminent scientists, Étienne Geoffroy Saint-Hilaire in Paris and Richard Owen in London, in the debate as to whether the platypus and echidna were really mammals, and whether they laid eggs. Geoffroy argued that they were egg-laying but could not possibly lactate; Owen argued that they lactated but could not possibly lay eggs. Because of these and many other aspects of their biology, monotremes featured prominently in debates about classification of animals and the transmutation of species, and involved many important scientists of the time. These arguments can only be understood in the context of the development of science in the late eighteenth and the nineteenth century, and how that was influenced by the social context. Early ideas of evolution, or transformism, were attractive to radical thinkers, whereas social conservatives were anxious to show that the boundaries between types of animals, just like the boundaries between social classes, were erected by God and could not be crossed.