62 THE ORIGIN OF SPECIES 



most in the same manner as does the vain endeavor 

 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 geologi* 

 cal periods," but in estimating them by years. When 

 geologists look at large and complicated phenomena, 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 1,000 feet of solid rock, as it became gradually dis- 

 integrated, would thus be removed 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 animals, which propagate their kind much more 

 slowly than most of the lower animals, that they have 



