"Though I should be very glad to take shelter under the convenient _Quien sabe_? the use of which I have suggested to you, I could not avoid, at the time I was in Mexico, putting my isolated facts together, and feeling inclined to believe that this country had not only been inhabited in extremely remote times, when the valley bore a very different aspect from that which it now exhibits, or which tradition gives it, but that the extinct race of enormous animals, whose remains would seem, in the instance I have cited, to be coeval with the undated works of man, may have been subjected to his will, and made instrumental, by the application of their gigantic force, to the transport of those vast masses of sculptured and chiselled rock which we marvel to see lying in positions so far removed from their natural site. "The existence of these ancient paved causeways also, not only from their solid construction over the flat and low plains of the valley, but as they may be traced running for miles over the dry table-land and the mountains, appears to me to lend plausibility to the supposition; as one might inquire, to what end the labour of such works, in a country where beasts of burden were unknown? "But I leave this subject to wiser heads and bolder theorists. Had the Mammoth of Chapingo been discovered with a ring in his nose, or a bit in his mouth, a yoke on his head, or a crupper under his tail, the question would have been set at rest. As it is, there is plenty of room for conjecture and dispute."[13] With respect to the great extinct Mammalia of South America, we find Mr Darwin, to whom we are indebted for our knowledge of so many of them, continually expressing his wonder at the comparatively modern era of their existence. After having enumerated nine vast beasts, which he found imbedded in the beach at Bahia Blanca, within the space of 200 yards square, and remarked how numerous in kinds the ancient inhabitants of the country must have been, he observes that "this enumeration belongs to a very late tertiary period. From the bones of the _Scelidotherium_, including even the kneecap, being entombed in their proper relative positions, and from the osseous armour of the great armadillo-like animal being so well preserved, together with the bones of one of its legs, we may feel assured that these remains were fresh and united by their ligaments when deposited in the gravel with the shells. Hence we have good evidence that the above-enumerated gigantic quadrupeds, more different from those of the present day than the oldest of the tertiary quadrupeds of Europe, lived whilst the sea was peopled with most of its present inhabitants."[14]