XXXIT PREFACE. 
mental industrial arts, including a certain amount of metallurgical knowl- 
edge. Evidence has been traced of a progressive desiccation throughout 
long climatic cycles in whose favorable extremes civilizations flourished 
which disappeared in the arid extremes. And that the climatic conditions 
under which these civilizations vanished gave rise to very early migrations 
and to a constructive reaction upon the outside world would seem to follow 
from the early appearance, in Babylonia and Egypt and in the late stone 
age in Europe, of wheat and barley and of breeds of domestic animals which 
Dr. Duerst identifies with those first established on the Transcaspian oases. 
I have in Chapter IV attempted to show that Central Asia was, from 
one of the epochs of the glacial period onward, isolated from Africa and 
Europe and that, excepting the elements of the lowest generalized form of 
human culture, all its cultural requirements were necessarily evolved and 
differentiated within the region of isolation. Before the supposedly Cen- 
tral-Asian Sumerians fused with the Semites on the Euphrates they had been 
trained in a struggle with nature which had culminated in the ability to 
conceive and execute great undertakings, as shown in the work of control- 
ling the great river. Their field of thought was doubtless confined largely 
to economic effort and organization. Into the fusion, the contemplative 
nomadic shepherd Semites brought a new range of speculative thought, 
and out of the union arose the highly developed Babylonian civilization. 
And to the extent that this entered into the origins of preclassic Af’gean and 
Mycenezan cultures, so far did it carry the contribution of the fundaments 
of civilization from the Central-Asian oases to the Mediterranean. 
The earlier reactions of the oasis cultures on the outside world were, 
therefore, both as regards migrations and ideas, essentially constructive in 
character. The later and greater migrations were of a different character. 
The growth of great nomadic populations, towhose outward movement these 
were due, could not have begun until after the development of the art of 
breeding the animals upon the possession of which alone life on the arid 
plains of Asia depended. I have shown, in the chapter on migrations, that 
during climatic conditions which depopulated the oases the grasses of the 
arid plains would permit the expansion and differentiation of nomadic shep- 
herd peoples till all Central Asia should be occupied, and that later there 
came a time when—in the progressive desiccation through an arid extreme of 
a climatic cycle and some thousands of years after the beginning of domes- 
tication and breeding of animals—the populations, swollen to the limit of 
the supporting capacity of the pasturage, would be forced to seek outlets 
into more favored regions. 
The great continental unrest which variously affected different parts 
of the West, being caused by the decreasing capacity of the pasturage to 
