Assuming that in its circumpolar origin the North Temperate flora was fairly homogeneous, it would meet in its centrifugal extension with a wide range of local conditions; these would favor the preservation of numerous species in some genera, their greater or less elimination in others.
The circumpolar lemmings of the genera Lemmus and Dicrostonyx are noticed in the article Lemming.
So terrestrial latitude would have to be 63 north for a lunar standstill north to be truly circumpolar today.
Dr Hart Merriam takes the broad point of view " that the whole of extratropical North America consists of but two primary life regions, a Boreal region, which is circumpolar,;and a Sonoran or Mexican tableland region which is unique."
The land mammals of Greenland are decidedly more American than European; the musk-ox, the banded lemming (Cuniculus torquatus), the white polar wolf, of which there seems to have been a new invasion recently round the northern part of the country to the east coast, the Eskimo and the dog - probably also the reindeer - have all come from America, while the other land mammals, the polar bear, the polar fox, the Arctic hare, the stoat (Mustela erminea), are perfectly circumpolar forms. The species of seals and whales are, if anything, more American than European, and so to some extent are the fishes.