Ans: The cooling climate of the tertiary period led to the huge glaciers at the poles. The mountains along with the newly formed Himalayas and alps were also covered with glaciers. The huge amount of water locked up in the sea lowered the level of sea and land bridges that appeared between Asia and North America, Great Britain and Europe, and South Asia and Borneo. This enables the migration of both plants and animals across these land bridges. The tertiary period which had begun hot and humid ended with a cold dry Icy age.