That being so, it is manifest that an accurate knowledge of rat-fleas, their forms, their habits, and their life-history may prove of great importance. Three species of the genus _Mus_ follow quickly in the wake of civilized man and establish themselves all over the globe. They may all be looked upon as more or less domestic animals. The house-mouse (_M. musculus_) is familiar everywhere. The old black rat (_M. rattus_) chiefly infests ships and seaports. The brown rat (_M. norwegicus_) is the most aggressive and distinctive. But all three, by accidental transference from port to port in ocean-going vessels, have become distributed over the world. Their fleas, to a limited extent, have become distributed with them. In connection with the spread of plague these three small rodents are of prime importance; and not less important are the fleas which are parasitic on them. In California, the ground-squirrel (_Otospermophilus beecheyi_) has been proved to play an important part in plague infection; and a full account of its fleas, and of experiments in transferring rat-fleas to squirrels and squirrel-fleas to rats, has been published by American naturalists. In 1903 Dr Blue, who was in charge of measures for suppressing plague at San Francisco, observed that an epidemic disease was killing the ground-squirrels in the country round San Francisco Bay. It was shown, somewhat later, that the mortality among the squirrels was caused by plague, and there can be little doubt that it was transferred from rats to squirrels. In harvest time rats migrate to the fields and use the same runs and holes as the squirrels. Under these conditions a transfer of fleas from rats to ground-squirrels is almost certain to ensue. Two species of flea have been recorded from the Californian ground-squirrel, and both are parasites of rats. The chain of evidence is really complete, for those who have made a business of hunting ground-squirrels testify to the readiness with which fleas will leave a dead squirrel and bite a human being. In the records of plague in California there are several cases in which there seems to be very little doubt that the disease resulted from handling plague-infected squirrels.