26 EVOLUTION 



rocks came into existence gradually and through the 

 operation of a supposed natural cause. 



A great advance upon Werner's theory was made 

 by Hutton, who, observing the formation of strata 

 at the present day from the sediment washed down 

 by rivers,; concluded that the ancient strata were 

 deposited in the same manner. Since,; by the long 

 continuation of this process the continents must 

 gradually become reduced to the level of the sea, 

 Hutton supposed that at long intervals of time the 

 action of subterranean heat came into play, and 

 fresh continents were upheaved, a process accom- 

 panied by the outpouring of the igneous rocks, 

 the true origin of which he had duly recognised. 

 In this theory a hypothetical cause still survives, since 

 we have no actual experience of vast upheavals of the 

 kind which Hutton supposed to have taken place. 

 Lyell showed that such slight changes of level as are 

 known to be in progress at the present day, especially 

 in association with the phenomena of earthquakes, 

 might, if continued over a long series of ages, give 

 rise to the necessary amounts of elevation. Lyell also 

 pointed out a number of subsidiary causes of dis- 

 integration and deposition of strata of the kind which 

 can still be seen in operation at different parts of the 

 earth's surface. At the present time it is sometimes 

 thought that Lyell went a little too far in his cham- 

 pioriship of the cause of uniformity. Lyell supposed 

 that the agencies which may now be everywhere ob- 

 served in operation, such as rain and rivers, the sea, 

 volcanoes and earthquakes, were sufficient to account 



