Complex sentence: Meanwhile, Australia, Melanesia and the Wallacea are united by a large share of similar animals, but few of these occur farther into the Pacific--but on the other hand, much of the Polynesian fauna is related to that of Melanesia.
Split into simpler sentences: Meanwhile, Australia, Melanesia and the Wallacea are united by a large share of similar animals, but few of these occur farther into the Pacific. On the other hand, much of the Polynesian fauna is related to that of Melanesia.