yet too ignorant of the post-pliocene and newer pliocene fauna 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 recent period, that continent was peopled with large kangaroos, and other herbivorous, and carnivorous marsupials, of species long since extinct, their remains having been discovered in ossiferous caverns. The preoccupaney of the country by such indigenous tribes may have checked the development of the placental rodents and cheiroptera, even were we to concede the possibility of such forms being convertible by variation and progressive development into higher grades of mammalia.
Imperfection of the geological record.
When treating in the 8th Chapter[1] 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 destroyed by rivers and the waves 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 for ever from our observation by mountain masses, thousands of feet thick, piled over them.
Mr. Darwin has truly said that the fossiliferous rocks known to geologists consist, for the most part, of such as
- ↑ Page 144 to 149.
