How did fossil horses provide evidence for evolution in Thomas Henry Huxley’s American lectures?

During his 1876 tour of the United States, Thomas Henry Huxley utilized the fossil record of horses found in North America to provide what he termed the "Demonstrative Evidence of Evolution."1

By presenting the chronological succession of these fossils, Huxley illustrated a clear, observable lineage that supported the theory of evolution. This evidence was particularly powerful because it offered a tangible, non-speculative sequence of anatomical changes over time, which helped to solidify the scientific case for evolutionary theory in the public eye.1