274 THE ORIGIN OF SPECIES 



formations in different parts of Great Britain; and this is the 

 result: 



Feet. 

 Palaeozoic strata (not including igneous beds) .... 57, 154 



Secondary strata 13,190 



Tertiary strata 2,240 



— making altogether 72,584 feet; that is, very nearly thirteen 

 and three-quarters British miles. Some of the formations, which 

 are represented in England by thin beds, are thousands of feet 

 in thickness on the Continent. Moreover, between each succes- 

 sive formation we have, in the opinion of most geologists, blank 

 periods of enormous length. So that the lofty pile of sedimentary 

 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 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 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 cer- 

 tain rivers, relatively to their areas of drainage, that 1,000 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 

 if halved or quartered it is still very surprising. Few of us, how- 

 ever, know what a million really means. Mr. Croll gives the fol- 

 lowing illustration: Take a narrow strip of paper, eighty-three 

 feet four 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, repre- 

 sented as it is by a measure utterly insignificant in a hall of the 

 above dimensions. Several eminent breeders, during a single life- 

 time, have so largely modified some of the higher animals, which 

 propagate their kind much more slowly than most of the lower 



