Peisker's description of the Scythian invasions of Irania[678] may be taken as typical of the whole area, and explains the complexity of the ethnological problems. The steppes and deserts of Central Asia are an impassable barrier for the South Asiatics, the Aryans, but not for the North Asiatic, the Altaian; for him they are an open country, providing him with the indispensable winter pastures. On the other hand, for the South Asiatic Aryan these deserts are an object of terror, and besides he is not impelled towards them as he has winter pastures near at hand. It is this difference in the distance of summer and winter pastures that makes the North Asiatic Altaian an ever-wandering herdsman, and the grazing part of the Indo-European race cattle-rearers settled in limited districts. Thus, while the native Iranian must halt before the trackless region of steppes and deserts and cannot follow the well-mounted robber-nomad thither, Iran itself is the object of greatest longing to the nomadic Altaian. Here he can plunder and enslave to his heart's delight, and if he succeeds in maintaining himself for a considerable time among the Aryans, he learns the language of the subjugated people and, by mingling with them, loses his Mongol characteristics more and more. If the Iranian is now fortunate enough to shake off the yoke, the dispossessed iranised Altaian intruder inflicts himself upon other lands. So it was with the Scythians. Leaving their families behind in the South Russian steppes, the Scythians invaded Media _c._ B.C. 630, and advanced into Mesopotamia as far as Egypt. In Media they took Median wives and learned the Median language. After being driven out by Cyaxares, on their return, some 28 years later, they met with a new generation, the offspring of the wives and daughters whom they had left behind, and slaves of an alien race. A hundred and fifty years later Hippocrates remarked their yellowish red complexion, corpulence, smooth skins, and their consequent eunuch-like appearance--all typically Mongol characteristics. Hippocrates was the most celebrated physician and natural philosopher of the ancient world. His evidence is unshakeable and cannot be invalidated by the Aryan speech of the Scythians. Their Mongol type was innate in them, whereas their Iranian speech was acquired and is no refutation of Hippocrates' testimony. On the later Greek vases from South Russian excavations they already appear strongly demongolised and the Altaian is only suggested by their hair, which is as stiff as a horse's mane--hence Aristotle's epithet [Greek: euthytriches]--the characteristic that survives longest among all Ural-Altaian hybrid peoples.