When thirty-six years old he was elected a member of the Institute.
In 1818 he began to discuss philosophical anatomy, the doctrine of homologies; he also studied the embryology of the mammals, and was the founder of teratology.

It was he who discovered the vestigial teeth of the baleen whale and those of embryo birds, and the bearing of this on the doctrine of descent must have been obvious to him.
As early as 1795, before Lamarck had changed his views as to the stability of species, the young Geoffroy, then twenty-three years old, dared to claim that species may be only "_les diverses degenerations d'un meme type_." These views he did not abandon, nor, on the other hand, did he actively promulgate them.

It was not until thirty years later, in his memoir on the anatomy of the gavials, that he began the series of his works bearing on the question of species.

In 1831 was held the famous debates between himself and Cuvier in the Academy of Sciences.

But the contest was not so much on the causes of the variation of species as on the doctrine of homologies and the unity of organization in the animal kingdom.
In fact, Geoffroy did not adopt the views peculiar to his old friend Lamarck, but was rather a follower of Buffon.