单项选择题

TEXT B
Whenever two or more unusual traits or situations are found in the same place, it is tempting to look for more than a coincidental relationship between them. The high Himalayas and the Tibetan plateau certainly have extraordinary physical characteristics and the cultures that are found there are also unusual, though not unique. However there is no intention of adopting Montesquieu’s view of climate and soil as cultural determents. The ecology of a region merely poses some of the problems faced by the inhabitants of the region, and while the problems facing a culture are important to its development, they do not determine it.
The appearance of the Himalayas during the late Tertiary Period and the accompanying further raising of the previously established rages had a marked effect on the climate of the region. Primarily, of course, it blocked the Indian monsoon from reaching Central Asia at all. Secondly, air and moisture from other directions were also reduced.
Prior to the raising of the Himalayas, the land now forming the Tibetan uplands had a dry, continental climate with vegetation and animal’s life similar to that of much of the rest of the region on the same parallel, but somewhat different from that of the areas farther north, which were already drier^①. With the coming of the Himalayas and the relatively sudden drying out of the region, there was a severe thinning out of the animal and plant population. The ensuing incomplete Pleistocene glaciations had a further thinning effect, but significantly did not wipe out life in the area. Thus after the end of the glaciations there were only a few varieties of life extant from the original continental species. Isolated by the Kunlun range from the Tarim basin and Turfan depression, species that had already adapted to the dry steppe climate, and would otherwise have been expected to flourish in Tibetan, the remaining native fauna and flora multiplied^②. Armand de scribed the Tibetan fauna as not having great variety, but being "striking" in the abundance of the particular species that are present. The plant life is similarly limited in variety, with some observers finding no more than seventy varieties of plants in even the relatively fertile Eastern Tibetan valleys, with fewer than ten food crops. Tibetan "tea" is a major staple, perhaps replacing the unavailable vegetables.
The difficulties of living in an environment at once dry and cold, and populated with species more usually found in more hospitable climates, are great. These difficulties may well have influenced the unusual polyandrous societies typical of the region. Lattimore sees the maintenance of multi-husband households as being preserved from earlier forms by the harsh conditions of the Tibetan uplands, which permitted no experimentation and "froze" the cultures that came there. Kawakiwa, on the other hand, sees the polyandry as a way of easily permitting the best householder to become the head husband regardless of age. His de tailed studies of the Bhotea village of Tsumje do seem to support this idea of polyandry as a method of talent mobility in a situation where even the best talent is barely enough for survival.
In sum, though arguments can be made that a pre-existing polyandrous system was strengthened and preserved (insofar as it has been) by the rigors of the land, it would certainly be an overstatement to lay causative factors of any stronger nature to the ecological influences in this case^③.
The purpose of the passage is to ______.