Darwin built up an irrefutable argument that species have changed and originated from other species, and that evolution has occurred. That he should have been able to do so from such few data is a mark of genius, for at the time when he worked out his conclusions, none of the cases had been discovered which would now be used as the most striking examples with which to illustrate the fact and the course of evolution. Chief among these are the beautiful series of fossils which reveal the evolution of the ammonites or of the horses, step by step and those which represent the precursors of the various classes and groups of vertebrates such as Archaeopteryx, or Pithecanthropus.

Fossils found in successive layers of the earth's crust show that the plant and animal life of the world has continuously changed. There has been a succession of floras and faunas, and the changes which the related groups of plants and animals have undergone are due to evolution.

The Cambrian is the earliest geological period from which well preserved fossils survive; its rocks contain seaweeds, worms, shells, trilo-bites and graptolites, but no land plants, insects or vertebrates.

Fish-like vertebrates first appeared in the Ordovician, true fish and land plants in the Silurian, amphibians in the Devonian, insects and reptiles in the Carboniferous, mammals in the Trias, birds in the Jurassic, flowering plants in the Cretaceous, and man in the Pleistocene period. Conversely, graptolites became extinct in the Carboniferous, trilobites in the Permian, pteridosperms in the Trias, dinosaurs in the Cretaceous.

The strata of the later geological periods do not contain many plants and animals of earlier periods. New forms were continually appearing while other forms became extinct.

Ammonites are an extinct group of shellfish, related to modern squids, octopus, and nautilus, and usually having a spirally coiled external shell. Their evolution can be illustrated by that of a line represented successively by two families, the Liparoceratidae and Amaltheidae, which lived in the seas of the Lower Jurassic Period (Lias) about 165 million years ago. The time-span of the two families is three to four million years. The succession of forms naturally shows the order of their appearance in the Liassic rocks. The sequence shows not only evolutionary change but divergence into different evolutionary lines.

Liparoceras cheltiense is the root stock of the whole evolutionary sequence; its whorls are large and thick, and the inner whorls are almost entirely overlapped by the outer whorls. This type of ammonite persisted with little change up to the middle of zone C (L. nautiliforme).

At the level of the Upper Lias the whole group of ammonites became extinct. The reasons are not fully known, but in Britain extinction coincided with a sudden change in the types of sediment being laid down in all areas. It seems likely that the ammonites then existing were not able to evolve sufficiently rapidly to become adapted to their environment and so to survive. (L. S. Spath.)

The evolutionary history of the horse can be traced step by step, showing its descent from a small browsing animal no bigger than a fox- terrier.

The earliest horses lived in the Eocene period about 70 million years ago, and were about the size of a fox-terrier, with a shoulder height of eleven inches. Hyracotherium (or Eohippus) had four functional toes on each forefoot and three on the hind feet, probably adapted for life on soft marshy land. The molar teeth were low-crowned and suited only to browsing on soft herbage.

In the Oligocene period, about 45 million years ago, horses had increased in size and had a shoulder height of two feet. Miohippus had three toes on each foot with the central toe slightly larger than the other two. The low-crowned teeth and three-toed feet suggest they were still forms that lived in forests browsing on soft vegetation.

Horses of the Pliocene period, about 10 million years ago, had increased further in size and were about four feet high at the shoulder. They had high-crowned teeth closely resembling those of the modern horse Equus, and suited to grazing. The feet had a single large toe forming a hoof on each foot, and were adapted for running on hard ground. The side toes were still further reduced.

The genus Equus first appeared about a million years ago at the beginning of the Pleistocene period in North America, whence it spread rapidly to every continent except Australia. These animals became extinct in North America where they were re-introduced by man. The different species can be distinguished as true horses, zebras, and asses.