UNIFORMITY OF CHANGE 435 



time, is effected, may be said to move about, visiting and 

 revisiting different tracts in succession. 



To make still more clear the supposed working of this 

 machinery, I shall compare it to a somewhat analogous case 

 that might be imagined to occur in the history of human 

 affairs. Let the mortality of the population of a large 

 country represent the successive extinction of species, and 

 the births of new individuals the introduction of new species. 

 While these fluctuations are gradually taking place every- 

 where, suppose commissioners to be appointed to visit each 

 province of the country in succession, taking an exact 

 account of the number, names, and individual peculiarities 

 of all the inhabitants, and leaving in each district a register 

 containing a record of this information. If, after the com- 

 pletion of one census, another is immediately made on the 

 same plan, and then another, there will at last be a series 

 of statistical documents in each province. When those be- 

 longing to any one province are arranged in chronological 

 order, the contents of such as stand next to each other will 

 differ according to the length of the intervals of time be- 

 tween the taking of each census. If, for example, there are 

 sixty provinces, and all the registers are made in a single 

 year and renewed annually, the number of births and deaths 

 will be so small, in proportion to the whole of the inhabit- 

 ants, during the interval between the compiling of two con- 

 secutive 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 dis- 

 crepancy. 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 colonised by persons migrating 

 from surrounding districts. 



