Charles Darwin is most famous for his study of finches on the Galapagos Islands. He observed that the different islands had different types of finches, each with a unique beak shape that was adapted to the specific food sources available on that island. This observation was a key piece of evidence for his theory of evolution by natural selection.

However, Darwin's research was much broader than just finches. He studied a wide variety of organisms, including:

* Barnacles: He spent years studying barnacles, meticulously classifying and describing different species. This work helped him develop his understanding of variation and adaptation.

* Earthworms: Darwin was fascinated by earthworms and their role in soil formation. He wrote a book about them, "The Formation of Vegetable Mould, Through the Action of Worms."

* Plants: Darwin studied plants extensively, focusing on their pollination, seed dispersal, and other aspects of their biology. He even experimented with cross-pollination and self-pollination in his own garden.

* Domesticated animals: Darwin observed how humans had selectively bred animals over generations to produce desired traits. He saw parallels between this artificial selection and natural selection in nature.

So while finches are the most iconic example of Darwin's studies, his research encompassed a wide range of organisms, contributing to his groundbreaking theory of evolution.