1. Adaptation to the environment: Darwin observed that organisms in different environments had evolved specific adaptations that allowed them to survive and thrive in those conditions. For example, he noticed that plants and animals on islands often had unique traits that differentiated them from their counterparts on the mainland.
2. Variation within species: Darwin also observed considerable variation within a single species. He noted that individuals within the same population could differ in size, shape, and other characteristics. This variation played a role in his understanding of natural selection.
3. Geographical distribution: Darwin studied the geographical distribution of plants and animals and found that species often had specific ranges where they were found. He observed that related species often occupied distinct but adjacent regions, leading to the concept of geographical speciation.
4. Fossils and extinction: Darwin's exploration of the fossil record provided evidence of extinct species and highlighted the concept of geological succession. He found that certain fossils were characteristic of specific geological periods and that some species had become extinct over time.
5. Similarities between species: Darwin noted that even though species might be different, they often shared certain fundamental characteristics. He proposed that these similarities could be explained by common descent from a shared ancestor.
Darwin's observations, coupled with his theory of natural selection, formed the foundation of evolutionary biology and revolutionized our understanding of the diversity and interconnectedness of life on Earth.