The three species of the genus Mus mentioned above follow in the wake of civilised man and may be looked upon as more or less domestic animals. Moreover, when they appear in any new locality, they seem to possess the power of eliminating the previously established native species of rats and mice, and they thus obtain for themselves a wider field and the opportunity for a rapid permanent increase in number of individuals. Of the three, Mus rattus alone habitually infests ships, but by accidental transference in vessels from port to port all three have become distributed throughout the world. Australia is especially rich in peculiar local species of rats, abundantly distinct from, but to some extent resembling, Mus rattus. This circumstance renders it not improbable that on occasion animals from Australia may have been incorrectly identified with that cosmopolitan species.