He believed in the underlying unity of organismal design, and the possibility of the transmutation of species in time, amassing evidence for his claims through research in comparative anatomy, paleontology, and embryology.

By a law passed in June 1793, Geoffroy was appointed one of the twelve professors of the newly constituted Muséum National d'Histoire Naturelle, being assigned the chair of zoology.

The two friends wrote together five memoirs on natural history, one of which, on the classification of mammals, puts forward the idea of the subordination of characters upon which Cuvier based his zoological system.

It was in a paper entitled Histoire des Makis, ou singes de Madagascar, written in 1795, that Geoffroy first gave expression to his views on the unity of organic composition, the influence of which is perceptible in all his subsequent writings; nature, he observes, presents us with only one plan of construction, the same in principle, but varied in its accessory parts.

In March of the following year Napoleon, who had already recognized his national services by the award of the cross of the legion of honor, selected him to visit the museums of Portugal, for the purpose of procuring collections from them, and in the face of considerable opposition from the British he eventually was successful in retaining them as a permanent possession for his country.

[5] In 1809, the year after his return to France, Geoffroy was made professor of zoology at the faculty of sciences at Paris, and from that period he devoted himself more exclusively than before to anatomical study.

[citation needed] When, in 1830, Geoffroy proceeded to apply to the invertebrata his views as to the unity of animal composition, he found a vigorous opponent in Cuvier, his former friend.

With Johann Wolfgang von Goethe he held that there is in nature a law of compensation or balancing of growth, so that if one organ take on an excess of development, it is at the expense of some other part; and he maintained that, since nature takes no sudden leaps, even organs which are superfluous in any given species, if they have played an important part in other species of the same family, are retained as rudiments, which testify to the permanence of the general plan of creation.

[18] Rue Geoffroy Saint-Hilaire [fr] is a street in the 5ème arrondissement, Paris near the Jardin des Plantes and Muséum national d'histoire naturelle.

French author Honoré de Balzac dedicated his novel Le Père Goriot to Saint-Hilaire, "as a tribute of admiration for his labors and his genius."