Speciation has proceeded at a normal rate, but owing to the relatively small size of the land mass of New Zealand and the lack of variations in the major habitats, the number of species in any group is not great. During the Pleistocene period when glaciation occurred in parts of New Zealand, it is probable that many endemic species then became extinct. The present-day absence of many groups of tropical or subtropical insects could also be due to their extinction in the glaciation periods.