THE EVIDENCE FOR ORGANIC EVOLUTION 513 



those now living. There are many gaps, and many types have 

 become extinct, but the geologic succession shows the " grand 

 course of evolution " so far as it can be ascertained from the fossil 

 record. 



Special Examples of Geologic Succession. — Invertebrates.^ 

 There are many instances among invertebrate animals, par- 

 ticularly in molluscs and brachiopods (Fig. 275) where the suc- 

 cession of fossils shows the course of evolution. In the interpreta- 

 tion of such a series one must first ascertain the order in which the 

 strata where originally laid down and thus the relative age of the 

 fossils they contain. A striking example is found in the extinct 

 forms called ammonites. These were cephalopod molluscs (c/. Fig. 

 117, p. 240) that flourished in Palaeozoic times. Beginning with 

 the so-called nautiloid type in the Ordovician, the fossils show a 

 succession that culminated in greatly specialized shells and such a 

 wealth of species that the ammonites were for a time the dominant 

 forms of marine life. Later, the ammonites became extinct, while 

 the ancient nautiloid type persisted as the genus Nautilus, which is 

 still living in the Philippine region and the Indian Ocean. 



Thus the Nautilus has preserved its general organization without 

 substantial changes throughout a span of geologic time sufficient 

 for the evolution of the entire series of vertebrate animals. This is 

 a very exceptional case, since the vast majority of animals seem to 

 have been modified with the passing ages, or to have become 

 extinct. 



The Horses. — The fossil record of horses is probably more com- 

 plete than that of any other mammalian type. Hence, it is almost 

 always cited in elementary accounts of the evidence for evolution 

 (Fig. 276). The series shown occurs in the fossil beds of the 

 Western United States, where horses flourished for a long period 

 but became extinct during the Pleistocene. In more recent times 

 the members of the horse family, including the horses, wild asses, 

 and zebras, have been found only in Europe, Asia, and Africa. 

 The " wild " horses of the Americas in recent centuries are the 

 descendants of horses that escaped from the settlers. 



In further explanation of the figure, it seems probable that the 

 ancestors of the earliest American horses, like the " dawn horse " 

 Eohippus, must have originated in another region and migrated 

 to North America, since they appear suddenly in the fossil record 

 without antecedent types. Central Asia is suspected as the place 



