We may first notice here a necessary limitation to our knowledge. The dry land of this age was of greater dimensions than at present. A large portion of what then was land is consequently now under the sea or deeply buried in alluvial deposits. Hence if any men of this age lived near the borders of the ocean, their remains must now be inaccessible, and the relics which we find must be those of inland tribes or of those who were driven inland by the encroachments of the waters. Our means of information are thus limited, and we must be prepared to admit that there may have been in this age great and populous communities of which we can have no record, at least of a geological character. Hence if we should find remains of only rude races of men, we should not be justified in assuming that all the peoples of the palanthropic age were of this character, more especially if we can find any indications that the men whose remains are accessible to us, though rude themselves, may have belonged to more advanced races. [Illustration: FLINT HACHE OF THE ANCIENT OR CHELLEAN TYPE, AURILLAC (after Carthaillac)] The bones, implements and weapons, and _débris_ of the feasts of these primitive peoples are to be found principally in caves of residence or of sepulture,[15] and in the alluvia deposited by rivers, and in a few cases in rock fissures or marine gravels, into which remains were drifted, or in which they were deposited by water. Here, again, we have another limitation, for it is possible that large populations may have lived on plains or in forests in perishable structures, and, like some modern savages, may have disposed of their dead in such a way that their bones could not have been preserved. In such cases we can hope to obtain, and then very rarely, only stone implements and other imperishable relics. [15] Caverns, in relation to this subject, may be divided into those of residence, in which early men have lived and have left therein the _débris_ of their food, the ashes and cinders of their fires, and implements, &c.; those of sepulture, in which the bodies of the dead have been deposited; and those of inundation, into which the bodies of animals or men have been drifted by floods. The same cave may, however, exhibit these different conditions in the deposits on its successive floors. Thus men may have inhabited a cave for a time; it may next have been invaded by river floods depositing mud, and it may subsequently have been used for burial. Notwithstanding these limitations, however, it is wonderful that so much has been recovered from the ground by the diligence of collectors, and that the material thus obtained has proved so fertile in information respecting our long-perished ancestors.