The basic concepts of linguistics have tended to follow those of other sciences. In the 19th century, when the clarification of species and the study of their evolution dominated the natural sciences, linguistics was concerned with the parallel themes of comparison and genetic relationship. With the change in emphasis towards structure and function, apparent in a natural science like biology, and also in other fields such as sociology, linguistics also turned to a structural and descriptive study. Despite the many trends such a division must ignore, it therefore seems reasonable to separate modern linguistic history into two parts: the naturalist period (19th century), and the structuralist period (20th century).