Another problem on which no light has been thrown concerns the evolution of the human flea. It would be of great interest to know whether the present species has undergone modifications of form since it became a parasite of the human race; whether we inherited the species from our simian ancestors; or whether the flea of one of the lower mammals became parasitic on mankind. In the Old World this flea is essentially a parasite of man. It occurs only occasionally on other mammals. In America it certainly appears to occur more frequently on mammals, other than man, than it does in the Old World. Human fleas can propagate in deserted human dwellings. The larvæ find nourishment in any refuse that has been left behind, and the adult insect can apparently continue for some time to reproduce itself without a meal of any sort and certainly without human blood. Travellers in the East and in Africa have described how on entering huts in deserted villages they have found their clothing covered with myriads of fleas, sometimes ravenous, and at others weak from long fasting. The human flea is a good deal more select in the choice of a host than some other species. The cat-flea (_Ctenocephalus felis_) has been found not only on the cat, but also on the dog, tiger, leopard, goat, horse, rat, hedgehog, kangaroo, deer, guinea-pig, rabbit, and on man. Many of these were specimens collected in zoological gardens. Although when hungry and confined in a test-tube the human flea will readily bite a rat or a guinea-pig, it has been found that human fleas kept with no other food-supply than rats and guinea-pigs soon die off. When large numbers of human fleas were wanted for experiments in Bombay, guinea-pigs were used as traps to attract them. On one occasion two guinea-pigs placed in a house which had been vacant for some days, and in which fleas must have been short of food, failed to attract any of this species; while a man who entered the house shortly afterwards acted as an admirable trap. Those who have not had experience of the abundance and voracity of fleas in oriental countries can hardly believe the numbers of human fleas that may be captured by sending a bare-legged man into a deserted house and then picking the fleas off him. In one house 31 _P. irritans_ were taken on a man’s legs in a few minutes. In another house 84 _P. irritans_, 8 cat-fleas and 1 bird-flea were caught. In a third, 150 _P. irritans_ and 4 cat-fleas were captured in a short time.