Masterpieces of Science 



to revisit the same place until the expiration of 

 sixty years, there will then be an almost entire 

 discordance between the persons enumerated 

 in two consecutive registers in the same province. 

 There are, undoubtedly, other causes, besides 

 the mere quantity of time, which may augment 

 or diminish the amount of discrepancy. Thus, 

 at some periods a pestilential disease may have 

 lessened the average duration of human life ; or a 

 variety of circumstances may have caused the 

 births to be unusually numerous, and the popula- 

 tion to multiply; or a province may be suddenly 

 colonized by persons migrating from surround- 

 ing districts. 



These exceptions may be compared to the ac- 

 celerated rate of fluctuations in the fauna and 

 flora of a particular region, in which the climate 

 and physical geography may be undergoing an 

 extraordinary degree of alteration. 



But I must remind the reader that the case 

 above proposed has no pretensions to be regarded 

 as an exact parallel to the geological phenomena 

 which I desire to illustrate ; for the commissioners 

 are supposed to visit the different provinces 

 in rotation; whereas the commemorating pro- 

 cesses by which organic remains become fossil- 

 ized, although they are always shifting from 

 one area to the other, are yet very irregular 

 in their movements. They may abandon 

 and revisit many spaces again and again, be- 

 fore they once approach another district; and, 

 besides this source of irregularity, it may often 

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