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The aim of this paper is to give an account of the first discoveries in the field of symbiosis around 1880. At the time and until the definition of symbiosis parasitism remained an important framework. Individuals living in animal or plant symbiosis were often defined in relation to parasites. Albert-Bernhardt Frank and Anton De Bary went beyond this conception, cleverly suggesting a new approach by means of the concept of “living together.” For lichens this coexistence of two species in a single life form was easy to understand. Thus the close and lasting association of two radically different species constitutes the unity of one and the same organism. This concept of association was then extended to the case of animal symbiosis. Karl Brandt (1881) reoriented symbiosis toward mutual benefit, but Oscar Hertwig (1883) showed that it is merely an interrelation. Patrick Geddes (1882) and Brandt (1883) ran some experiments that make it possible to demonstrate scientifically the function of symbiotic algae in nutrition and respiration.