56 THE LAPSE OF TD1E. [Chap. X. 



idea of the time which has elapsed during their accumula- 

 tion. The consideration of these various facts impresses 

 the mind almost in the same manner as does the vain 

 endeavour to grapple with the idea of eternity. 



Nevertheless this impression is partly false. Mr. 

 Croll, in an interesting paper, remarks that we do not 

 err " in forming too great a conception of the length of 

 " geological periods," but in estimating them by years. 

 When geologists look at large and complicated phe- 

 nomena, and then at the figures representing several 

 million years, the two produce a totally different effect 

 on the mind, and the figures are at once pronounced too 

 small. In regard to subaerial denudation, Mr. Croll 

 shows, by calculating the known amount of sediment 

 annually brought down by certain rivers, relatively to 

 their areas of drainage, that 1000 feet of solid rock, as 

 it became gradually disintegrated, would thus be re- 

 moved from the mean level of the whole area in the 

 course of six million years. This seems an astonishing 

 result, and some considerations lead to the suspicion 

 that it may be too large, but even if halved or quartered 

 it is still very surprising. Few of us, however, know 

 what a million really means : Mr. Croll gives the 

 following illustration : take a narrow strip of paper, 83 

 feet 4 inches in length, and stretch it along the wall of 

 a large hall ; then mark off at one end the tenth of an 

 inch. This tenth of an inch will represent one hundred 

 years, and the entire strip a million years. But let it 

 be borne in mind, in relation to the subject of this work, 

 what a hundred years implies, represented as it is by a 

 measure utterly insignificant in a hall of the above 

 dimensions. Several eminent breeders, during a single 

 lifetime, have so largely modified some of the higher 



