252 THE LAPSE OF TIME. [CHAP. X. 



tary rocks in Britain gives but an inadequate idea of the time 

 which has elapsed during their accumulation. 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 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 1000 feet of solid 

 rock, as it became gradually disintegrated, 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 

 formed what well deserves to be called a new sub-breed. Few 

 men have attended with due care to any one strain for more than 

 half a century, so that a hundred years represents the work of 

 two breeders in succession. It is not to be supposed that species 

 in a state of nature ever change so quickly as domestic animals 

 under the guidance of methodical selection. The comparison 

 would be in every way fairer with the effects which follow from 

 unconscious selection, that is the preservation of the most useful 

 or beautiful animals, with no intention of modifying the breed ; 

 but by this process of unconscious selection, various breeds have 

 been sensibly changed in the course of two or three centuries. 



Species, however, probably change much more slowly, and 

 within the same country only a few change at the same time. 

 This slowness follows from all the inhabitants of the same 

 country being already so well adapted to each other, that new 



