Paleontology really got started though, with the publication of "Recherches sur les poissons fossiles" (1833–1843) by Swiss naturalist Louis Agassiz (1807–1873). He studied, described and listed hundreds of species of fossil fish, beginning the serious study into the lives of extinct animals. With the publication of the Origin of Species by Charles Darwin in 1849, the field got a theoretical framework. Much of the subsequent work has been to map the relationship between fossil and extant organisms, as well as their history through time.