448 IMPERFECTION OF THE GEOLOGICAL RECORD, chap. xxir. 



yet too ignorant of the post-pliocene and newer pliocene 

 fiuina of that part of the world, to be able to decide whether 

 the introduction of such forms dates from a remote geological 

 time. We know, however, that, before the Eecent period, that 

 continent was peopled with large kangaroos, and other her- 

 bivorous and carnivorous marsupials, of species long since 

 extinct, their remains having been discovered in ossiferous 

 caverns. The preoccupancy of the country by such indigenous 

 tribes may have checked the development of the placental 

 rodents and cheiroptera, even were we to concede the pos- 

 sibility of such forms being convertible b}' variation and 

 l^rogressive development into higher grades of mammalia. 



Imperfection of the geological record. 



When treating in the 8th Chapter* of the dearth of human 

 bones in alluvium containing flint implements in abundance, 

 I pointed out that it is not part of the plan of Nature to write 

 everywhere, and at all times, her autobiographical memoirs. 

 On the contrary, her annals are local and exceptional from 

 the first, and portions of them are afterwards ground into 

 mud, sand, and pebbles, to furnish materials for new strata. 

 Even of those ancient monuments now forming the crust of 

 the earth, which have not been destroj'ed by rivers and the 

 ■fwaves of the sea, or which have escaped being melted by 

 volcanic heat, three-fourths lie submerged beneath the ocean, 

 and are inaccessible to man ; while of those which form the 

 dry land, a great part are hidden forever from our obser- 

 vation by mountain-masses, thousands of feet thick, piled 

 over them. 



Mr. Darwin has truly said that the fossiliferous rocka 

 known to geologists consist, for the most part, of such as 



* Pages 144 to 149. 



