Charles observed in this voyage, that the landmasses were rising with the passage of time. He concluded this by observing the geological strata, marine and plant life, fossils and observing the variety of birds present on the islands of South America. Studying the mockingbirds and tortoises in the area, the theory of the origin of the species began to take root in his mind. Meanwhile Charles kept sending back specimens and letters describing his findings, which became greatly admired.