THE HISTORY OF ORGANISMS. 9 



believe that the time that has transpired since the first organ- 

 isms lived upon the earth is measured by millions rather than 

 by centuries of years, " tens of millions and not millions nor 

 hundreds of millions," as Mr. Walcott maintains.* Sufficient 

 evidence appeared to have convinced the earlier geologists of 

 this statement ; but the evidence is not direct testimony to 

 the fact of the great antiquity of the earth and its inhabitants. 

 The fact that fossils are in the solid rocks and that they 

 are different from the shells or hard parts of any organisms 

 now living, were facts well known long before the notion 

 of six thousand years was considered inadequate for the 

 history of the earth. But the opinion that the fossils 

 were the remains of organisms of no great antiquity, arid 

 that they had been buried by some great flood, some 

 extraordinary cataclysm, was held to be sufficient to explain 

 the brevity of the assumed time ; and the differences between 

 the fossils and the living forms were mysteries which were 

 simply not explained at all until about the beginning of the 

 present century. The general belief that cataclysms are pos- 

 sible, that antiquity is the great reservoir for the remarkable, 

 the extravagant, the unscientific, or the unknown, has been, 

 and is to some extent now, the common excuse for mistakes 

 made in interpreting the laws of nature. In the study of 

 rocks, we need to learn how to use them as measures of the 

 time-relations of the fossil contents. An analysis of the 

 classifications which have hitherto been made to express the 

 chronological relations of rocks will show us what the facts 

 are, how these facts have been interpreted, and how far these 

 interpretations are at present satisfactory. 



* Vice-Presidential Address, Section E, Am. Assoc. Adv. Sci., 1893. 



