ORGANIC EVOLUTION 495 



tricity, or external ones like winds and currents, or even direction 

 by wireless. Similarly, in the case of the doctrine of Evolution, 

 there is the Fact of Evolution as the historic series of events; the 

 Course of Evolution, for example, whether the land vertebrates 

 arose from fish-like ancestors, or birds from reptiles; and the 

 Causes of Evolution, or what made it take place. These three 

 aspects of the evolutionary process are separate though related 

 factors. They must be distinguished if there is to be any clear 

 thinking in these matters. 



To illustrate concretely, the fossil record shows that there were 

 originally only fish-like vertebrates (cf. Fig. 259). These are 

 connected by a series of fossils with the fishes of the present day. 

 Later there were amphibians in addition to the fishes, then rep- 

 tiles, followed by mammals, and birds. In each of these cases the 

 original types are connected with those of the present by the fossils 

 of subsequent strata. Such facts as these led the great paleontolo- 

 gist Cuvier (1769-1832) to acknowledge a fact that is evident 

 throughout the geologic record, i.e., that the animals of the past are 

 different from the animals of the present, save in exceptional cases, 

 and that the farther back one goes the greater are the differences. In 

 addition, there occur in the fossil record sijnthetic types which seem 

 to be intermediate between the different lines, like the toothed 

 birds (Fig. 279), which are " missing links " between birds and 

 reptiles; and the forms that suggest transitions from fishes to 

 amphibians, and from amphibians to reptiles. 



Such fossil records, together with the vast body of evidence 

 derived from the representatives of this group that are now hving, 

 constitute the evidence for evolution in vertebrate animals. If 

 the vertebrates have had such a history, one may distinguish the 

 fact that their ancestors were fish-like animals from which present- 

 day forms have originated by evolution, from the course that this 

 evolution has pursued in the origin of the various types, and from 

 the causes by which such evolutionary changes have been effected. 

 In general, it may be said that scientists regard the historic fact 

 of Organic Evolution as the most reasonable interpretation of the 

 evidence ; that the course of this evolution is apparent in many of 

 its broader aspects and in special cases, such as the horse and camel 

 families (Figs. 276 and 280) ; but that the causes are not yet known, 

 although various theories have been proposed. This does not 

 mean that the causes of Organic Evolution cannot be discovered, 



