At that time, Bergson had already made an extensive study of biology including the theory of fecundation (as shown in the first chapter of the "Creative Evolution"), which had only recently emerged, ca. 1885 – no small feat for a philosopher specializing in the history of philosophy, in particular Greek and Roman philosophy. He also most certainly had read, apart from Darwin, Haeckel, from whom he retained his idea of a unity of life and of the ecological solidarity between all living beings, as well as Hugo de Vries, from whom he quoted his mutation theory of evolution (which he opposed, preferring Darwin's gradualism). He also quoted Charles-Édouard Brown-Séquard, the successor of Claude Bernard at the Chair of Experimental Medicine in the Collège de France, etc.