hat form through history become
             filled with dissolved minerals that harden into fossils.
             Since different types of fossils are found at different layers of sedimentary rock,
             their presence at specific depths indicates the period in which the animals lived. The
             fact that certain species are found in certain layers is taken as evidence for evolution.
             In the 1870's, the paleontologist O.C. Marsh published a description of newly
             discovered horse fossils form North America. At the time, very few transitional fossils
             were known apart from Archeopteryx. The sequence of horse fossils that Marsh
             described was a striking example of evolution taking place in a single lineage. Here
             one could see the fossil species Eohippus transformed into an almost totally
             different-looking, and very familiar descendent, Equus, through a series of clear
             intermediates. Some years later, the American Museum of Natural
             ...