The sugar glider in Australia and the flying squirrel in North America are both small mammals with large expanses of loose skin between their forelimbs and hindlimbs, allowing them to glide from tree to tree within their respective forest habitats. These two species are not evolutionarily related but are very similar in their overall appearance. Darwin's explanation for their similarity is that

Each have adapted in very similar ways to similar habitats.

Explanation:

According to Darwin, given a similar set of conditions, two different organisms can adapt in the same direction. Here, the sugar glider inhabits Australia and the flying squirrel inhabits North America. There is a huge distance between their locations and also they are two different species with no direct evolutionary relation. However, because of the similar conditions present in their habitats, they developed same traits over the time. Both of them live in forested areas so they developed loose skin between their limbs which allowed them to glide freely between the trees.