338 PRINCIPLES OF ANIMAL BIOLOGY 



one can only assume that the course of development is brought down from 

 a common forebear. And when the present distribution of animals seems 

 to require that two groups should have sprung from a single antecedent 

 group, it is nevertheless only an assumption that they did thus diverge. 

 The conclusions stated above are plausible, but they are necessarily only 

 inferences. It is otherwise with the facts of paleontology. They 

 leave no doubt that animals were different in different periods; for 

 there are the remains to prove it. There is usually no need to argue 

 which are the earliest forms, for the age of the rocks in which they lie can 

 in general be ascertained in other ways. Often, unless there are long 

 gaps not bridged over, the course of evolution is clearly mapped out; 

 for in successive layers of rock are the remains that show by just what 

 steps a given kind of animal evolved. 



Paleontology occupies this favored position as evidence of evolution 

 only so long, however, as it is concerned merely with the fact of evolution 

 and with the course which evolution has taken. As soon as the facts of 

 fossils are used to make incursions into other fields, inference is necessary. 

 From a flat crowned tooth one may infer that the owner was a grazing 

 animal, but that is not proven. Restorations of extinct animals in the 

 flesh are inferences based on a knowledge of modern animals. From a 

 facial angle a certain degree of intelligence may be inferred, but only 

 inferred. The ridges on a bone may indicate heavy muscles, but they 

 lack something of proof. In particular, fossils offer no evidence regard- 

 ing the cause of evolution. It may be easy to devise plausible explana- 

 tions for the changes that animals have undergone; but for proof of the 

 correctness or incorrectness of these explanations facts have to be 

 gathered from other sources. 



References 



Dendy, a. Outlines of Evolutionary Biology. 



Lull, R. S. Evolution of the Horse Family. American Journal of Science, March, 



1907. 

 Lull, R. S. Organic Evolution. 

 Matthew, W. D. The Evolution of the Horse. Supplement to American Museum 



Journal, January, 1903. 

 Nicholson, H. A. Manual of Paleontology. 

 OsBORN, H. F. Men of the Old Stone Age. 

 ScHUCHERT, Charles. Historical Geology. 

 Williams, H. S. Geological Biology. 



