SIR CHARLES LYELL 221 



will be so small, in proportion to the whole of the inhabitants, during 

 the interval between the compiling of two consecutive documents, 

 that the individuals described in such documents will be nearly 

 identical; whereas, if the' survey of each of the sixty provinces 

 occupies all the commissioners for a whole year, so that they are 

 unable 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 population to 

 multiply ; or a province may be suddenly colonized by persons migrat- 

 ing from surrounding districts. 



These exceptions may be compared to the accelerated rate of fluc- 

 tuations 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 phe- 

 nomena which I desire to illustrate ; for the commissioners are sup- 

 posed to visit the different provinces in rotation; whereas the com- 

 memorating processes by which organic remains become fossilized, 

 although they are always shifting from one area to the other, are 

 yet very irregular in their movements. They may abandon and re- 

 visit many spaces again and again, before they once approach another 

 district; and, besides this source of irregularity, it may often happen 

 that, while the depositing process is suspended, denudation may take 

 place, which may be compared to the occasional destruction by fire or 

 other causes of some of the statistical documents before mentioned. 

 It is evident that where such accidents occur the want of continuity 

 in the series may become indefinitely great, and that the monuments 

 which follow next in succession will by no means be equidistant from 

 each other in point of time. 



If this train of reasoning be admitted, the occasional distinctness 

 of the fossil remains, in formations immediately in contact, would be 

 a necessary consequence of the existing laws of sedimentary deposi- 



