Modification of Earth' s Inhabitants 31 



to recognize, we find consistently, as William Smith had 

 pointed out, that the more recent fossils resemble the living 

 forms most, and that the oldest fossils resemble them least. 

 Moreover, we find in many cases a fairly continuous series 

 of forms connecting the oldest with the most recent.^ This 

 fact is called the geologic succession. 



Modification of Earth's Inhabitants 



In general terms then we find the observed facts regard- 

 ing the remains of plants and animals that lived in former 

 ages quite consistent with the idea that there has been in the 

 course of ages a gradual modification in the living inhabitants 

 of the earth. There is nothing, however, to show us that 

 the species living in one period were connected with those 

 of another period by actual descent. The succession may 

 have been, for all we know, like the replacement of candles 

 by oil lamps and of oil lamps by electric lights. Indeed, the 

 first scientist to make a systematic study of fossils for the 

 purpose of determining the kinds of animals they repre- 

 sented was quite convinced that there was no genetic con- 

 nection. Georges Cuvier, the founder of comparative 

 anatomy, had so thoroughly familiarized himself with the 

 details of structure of many backboned animals, especially 

 of their skeletons, that he claimed to be able to recognize 

 a bird or a mammal by one of its bones. In the course of 

 the extensive diggings in Paris in 1800, the workmen un- 

 covered large fossil bones, the. original sources of which they 

 could not know. Many guesses were made — they were the 

 remains of giants, perhaps, or monsters of a former time, 

 but nothing that people could recognize. These fossils were 

 brought to Cuvier, who at once recognized that they were 

 elephant bones! But there were no elephants in the neigh- 



^ A century ago it was necessary for the student of paleontology to 

 depend upon fossil types to give him the key to the chronological sequence 

 of the rock deposits. The accumulation of more and more facts has made 

 it possible to reverse this method and to determine the fossil sequence by 

 a study of the rock formations in which they were found. 



