The nearest allies of the human flea, which are found on various animals, are all inhabitants of the Old World. The indigenous fleas of America are only distant relatives of _Pulex irritans_. Our knowledge of the present and former distribution of this species is deplorably meagre. The many books of travel published in the early part of the nineteenth century contain hardly any records of fleas. The human flea is now cosmopolitan. Specimens identical with those from Europe are found almost everywhere. But it may be doubted whether this was the case before the great era of travel and steam began in last century. There is one strange and, indeed, inexplicable fact in connection with the distribution of this cosmopolitan species of flea. It is absent from the oases of the Sahara and the Haussa countries immediately to the south of the great desert. These countries have long been in communication with places where _Pulex irritans_ is known to abound. There is no natural barrier. The habits of the natives would encourage fleas to thrive, and other forms of human vermin are plentiful. There is, apparently, only one explanation that is forthcoming. It is suggested that the soil and climate in these regions of Africa are, for some reason, unsuited to fleas. In other parts of the Dark Continent, where there are European settlements, the human flea seems to thrive surprisingly well and to attack Europeans and natives, as well as wild and domestic animals. In those parts of Asia where there are European colonies and much intercourse between settlers and Orientals, _Pulex irritans_ is a well-established and thriving parasite. Unfortunately, there is no means of knowing whether this was the case among the native populations before European travellers and traders arrived. _Pulex irritans_ has, however, recently been found on the natives of German New Guinea living some 10,000 feet above sea-level and in great isolation. Seaports are everywhere infested with fleas.