Answer: alternative A.
Explanation: Fixism was a current theory during biology history, and it stated that species were always like they are at the present time, not being able to change during time. Darwin's evolutionary theory, developed at the 19th century, said the opposite: species were able to change and they still do it. One way to prove it was studying fossils and its morfologycal characteristics in order to link them to more recent species, and establish a taxonomical connection between them. The study of fossils is know as paleontology, and it this way it changed the naturalist's views.