272 Tilt: LAPSE OF TIME. Cuap. IX. 



illustration : take a narrow strip of paper, 83 feet 4 inclies in 

 length, and stretch it along the Avail 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 dimen- 

 sions. Several eminent breeders, during a single lifetime, have 

 so largely modified some of the higher animals, Avhich propa- 

 gate their kind much more slowly than most of the lower ani- 

 mals, that they have formed Avliat Avell deserve to be called 

 new sub-breeds. Few men have attended AA-ith due care to 

 any one strain for more than half a century, so that a hundred 

 years represent the Avork 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 Avould be in every AA'ay 

 fairer Avitli the results Avhich follow from unconscious selection, 

 that is, the preservation of the most useful or beautiful animals, 

 Avith no intention of modifying the breed ; but by this process 

 of unconscious selection, various breeds haA"e 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 avcU adapted to each other that ncAV 

 places in the polity of Nature do not occiir until after long in- 

 tervals, Avhen changes of some kind in the physical conditions 

 or through immigration haA^e occurred; and indiA^idual differ- 

 ences or variations of the right nature, by Avliich some of the 

 inhabitants might be better fitted to their new places under 

 the altered circumstances, might not at once occur. According 

 to the standard of years Ave have no means of determining 

 how long a period it takes to modify a species. Mr. Croll, 

 judging from the amount of heat-energy in the sun and from 

 the date Avhich he assigns to the last glacial epoch, estimates 

 that only sixty million years lia\'e elapsed since the deposi- 

 tion of the first Cambrian formation. This appears a A'ery 

 short period for so many and such great mutations in the forms 

 of life as have certainly since occurred. It is admitted that 

 many of the elements in the calculation are more or less doubt- 

 ful, and Sir "W. Thomson giA'Cs a Avide margin to the possible 

 age of the habitable Avorld. But, as Ave have seen, Ave cannot 



