On the second supposition, we have to face the fact that there is very little evidence from palaeontology of the former presence of the American type in Eurasia.
But there is nothing approaching the majesty, beauty, and swarming mass of the great mammalian life of Africa and, in a less degree, of tropical Asia; indeed, it does not even approach the similar mammalian life of North America and northern Eurasia, poor though this is compared with the seething vitality of tropical life in the Old World.
The compilation of W.M. MacGovern, The Early Empires of Central Asia, Chapel Hill 1939, is now quite antiquated.--An attempt to construct a model of Central Asian nomadic social structure has been made by E.E. Bacon, Obok, a Study of Social Structure in Eurasia, New York 1958, but the model constructed by B. Vladimirtsov and modified by O. Lattimore remains valuable.--For origin and early-development of Hsiung-nu society see O. Maenchen, K. Jettmar, B. Bernstam, Uchida Gimpu and many others.
p. 117: This analysis of tribal structure is based mainly upon my own research; it differs in detail from the studies by E. Bacon, Obok, a Study of Social Structure in Eurasia, New York 1958, B. Vladimirtsov, O. Lattimore's Inner Asian Frontiers of China, New York 1951 (2nd edit.)