But it must be remembered that this inference rests entirely on the fact that the district in question has been more or less explored, while the rest of the earth can hardly be said to have been explored at all, for anything prior to those Quaternary paleolithic implements which prove the existence of man already spread over nearly the whole of the habitable globe. Nor would the origin of the white race in Eur-Africa, even if it were established, help us to account for the existence of the Negro race on the other side of the Atlas and the Sahara, or of the yellow race in Eastern Asia, or of the American race. Indeed America may fairly compete with Eur-Africa for the honour of being the original seat of the human race, for the geological conditions and the animal fauna of the auriferous gravels of California point to the Calaveras skull and other numerous human remains and implements found in them being of Tertiary age, and quite possibly as old or even older than anything which has been found in Europe.[17] The wide diffusion of the same peculiar racial type over the whole continent of America down to Cape Horn, and its capability of existing under such different conditions of climate and environment, also point to its being an extremely ancient and primitive race, and the generic distinction between the apes and monkeys of the Old and New Worlds is a remarkable circumstance which is not accounted for by any monogenist theory of the origin of the order of quadrumana. [17] If Ameghino's discoveries of an anthropoid type in the Lower Eocene of Patagonia should be confirmed, it would incline the balance of evidence in favour of South America, or rather of the temperate zone of the southern hemisphere, as the most probable scene of the evolution of the quadrumana, including the human variety, from ancestral forms allied to the marsupials of the Secondary period. It is to be observed also, that although all American races have a certain peculiar type in common, still there are differences which show that secondary types must have existed from a very early period, intercrossing between which must have given rise to numerous varieties. Thus, according to Morton, dolichocephaly was most prevalent among the tribes who inhabited the eastern side of the continent facing the Atlantic both in North and South America, while brachycephaly prevailed on the western, side facing the Pacific. Great differences of colour and stature are also found often among contiguous tribes, and irrespective of latitude. On the whole, however, the American type approximates in many important particulars, such as colour, hair, and anatomical structure, more nearly to the yellow races of Eastern Asia than to any other, though it is a fairly open question which of the two may have been the earliest to appear in the immensely remote ages of the Tertiary period.