During his travels, Darwin made numerous observations and collected evidence that led him to propose a hypothesis about the way life changes over time. This hypothesis, now a theory, is regarding evolution or change over time. Evolution is the process by which modern organisms have descended from ancient organisms. The large variety of living organisms have descended from species similar but less adapted than themselves.

Darwin discovered that the traits of the birds and tortoises varied among the different islands in Galapágos. For instance, the structure of the tortoise indicates their adaptation to different habitats. Since Hood Island has sparse vegetation, the tortoise inhabiting the island has a longer neck in order to reach the vegetation. On the other hand, the tortoise living in the Isabela island, which has lush vegetation, has a shorter neck. In addition, Darwin also studied the Galapágos finches. He saw some remarkable differences among the beaks of the birds living in the different islands. Birds which pick seeds out of fruits have longer beaks, whereas birds who eat seeds from the ground have shorter beaks.

The theory of evolution explains how different species have evolved from ancient organisms. Despite its substantial explanation, it is still a theory because it has not been confirmed to a point where everyone in the scientific community and the whole society agrees to it.

Fossils are from the remains of ancient living things that are formed and buried under the sediments. These are traces that serve as clues to what the Earth was in the past.

Fossils that were unlike any living species must have been an extinct species. This shows that some species can die out, and not all organisms are suited to their environment. Life is diverse, but it is not always perfect. Organisms similar to the fossilized species did not exist because the species had died out.