Note 56 — THE MIGRATIONS OF MAMMALS.

[Note 56] p. 237.—Rats.

The brown rat (Mus decumanus) is much stronger than the black rat (Mus rattus), and gains an easy victory. The rivalry between the two species is one of the examples Darwin gives of his generalization that the struggle for existence is most severe between allied forms, and there seems no doubt that the brown rat has often ousted and will even devour the black rat. The latter is now rare in Britain. It should be noted, however, (1) that the two species are said sometimes to live together on board ship, (2) that cases have been recorded where the black rat returned and defeated its conquerors, and (3) that the black rat keeps more about houses, stables, and barns, is therefore more readily exterminated by man, whose efforts were doubtless increased when a second species appeared on the scene. The case is of some importance in connection with the generalization referred to above.