31S THE LAPSE OF TIME. 



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 *Mn 

 forming too great a conception of the length of " geological 

 periods,'^ but in estimating them by years. When geolo- 

 gists look at large and complicated phenomena, and then 

 at the figures representing several million years, the two 

 produce a totally dilferent 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 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, however, know what 

 a million really means: Mr. Croll gives the following illus- 

 tration: 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, 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 sup- 

 posed that species in a state of nature ever change so 

 quickly as domestic animals under the guidance of method- 

 ical selection. The comparison would be in every way 



