218 



ISLAND LIFE. 



[part t. 



denudation is effected by the waves of the ocean eating away 

 coast lines. This was once thought to be of more importance 

 than sub-aerial denudation, but it is now believed to be com- 

 paratively slow in its action.^ Whatever it may be, however, it 

 adds to the rate of formation of new strata, and its omission 

 from the calculation is again on the side of making the lapse of 

 time greater rather than less than the true amount. Even if a 

 considerable modification should be needed in some of the 

 assumptions it has been necessary to make, the result must still 

 show that, so far as the time required for the formation of the 

 known stratified rocks, the hundred million years allowed by 

 physicists is not only ample, but will permit of even more than 

 an equal period anterior to the lowest Cambrian rocks, as 

 demanded by Mr. Darwin — a demand supported and enforced 

 by the arguments, taken from independent standpoints, of 

 Professor Huxley and Professor Eamsay. 



Organic Modification dependent on Change of Conditions. — 

 Having thus shown that the physical changes of the earth's 

 surface may have gone on much more rapidly and occupied 

 much less time than has generally been supposed, we have now 

 to inquire whether there are any considerations which lead to 

 the conclusion that organic changes may have gone on with 

 corresponding rapidity. 



There is no part of the theory of natural selection which is 

 more clear and satisfactory than that which connects changes of 



ascertained fact, tliat the area of deposition is many times smaller than the 

 area of denudation. 



1 Dr. Croll and Professor Geikie have shown that marine denudation is 

 very small in amount as compared with sub-aerial, since it acts only locally 

 on the edge of the land, whereas the latter acts over every foot of the 

 surface. Mr. W. T. Blanford argues that the difference is still greater in 

 tropical than in temperate latitudes, and arrives at the conclusion that — 

 " If over British India the effects of marine to those of fresh- water denu- 

 dation in removing the rocks of the country be estimated at 1 to 100, I . 

 believe that the result of marine action will be greatly overstated" {Geo- 

 logy and Zoology of Abyssinia., p. 158, note). Now, as our estimate of 

 the rate of sub-aerial denudation cannot pretend to any precise accuracy, 

 we are justified in neglecting marine denudation altogether, especially as 

 we have no method of estimating it for the whole earth with any approach 

 to correctness. 



