Why are transitional species such as the ancestors of modern whales or horses crucial evidence for evolution?

Transitional species, like the ancestors of modern whales or horses, provide crucial evidence for evolution as they illustrate the gradual changes and adaptations that occur over time. These fossils reveal intermediate forms that bridge the gap between distinct groups, demonstrating how species can evolve from one form to another in response to environmental pressures. By documenting these evolutionary pathways, transitional species support the theory of evolution and highlight the continuity of life on Earth.