The Indian subcontinent has varied physical and climatic conditions and types of vegetation, stretching from the northern temperate Zone to the equator and embracing such contrasting regions as the Himalayas in the north and tropical sea-coast in the south. The affinity of fauna of the Siwalik beds and that of present-day Africa and India suggest that they were derived from common species which migrated from Europe and Central Asia which enjoyed warm climate at that time.

The advent of ice age in these regions must have forced many species to migrate southwards. Some of them reached India. Migration of fauna to India also took place from the eastern lands, which are now separated from the Bay of Bengal and also from the west.