220 CLASSICS OF MODERN SCIENCE 

 shells, corals, and fish, between the Silurian and Carboniferous rocks. 

 Previously the marine fauna of these last-mentioned formations 

 wanted the connecting links which now render the passage from the 

 one to the other much less abrupt. In like manner the Upper Mio- 

 cene has no representative in England, but in France, Germany, and 

 Switzerland it constitutes a most instructive link between the living 

 creation and the middle of the great Tertiary period. Still we must 

 expect, for reasons before stated, that chasms will forever continue 

 to occur, in some parts of our sedimentary series. 



Concluding remarks on the consistency of the theory of gradual 

 change with the existence of great breaks in the series. — To return to 

 the general argument pursued in this chapter, it is assumed, for rea- 

 sons above explained, that a slow change of species is in simultaneous 

 operation everywhere throughout the habitable surface of sea and 

 land; whereas the fossilization of plants and animals is confined to 

 those areas where new strata are produced. These areas, as we have 

 seen, are always shifting their position, so that the fossilizing process, 

 by means of which the commemoration of the particular state of the 

 organic world, at any given time, is efifected, 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 extinc- 

 tion of species, and the births of new individuals the introduction of 

 new species. While these fluctuations are gradually taking place 

 everywhere, 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 informa- 

 tion. If, after the completion 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 belonging 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 between 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 



