Mosquitoes and gnats are very abundant in many parts of Europe, and the forms belonging to the genus Anopheles, which carry the germ of malaria, are widely distributed. In parts of the Mediterranean area their presence is associated with the prevalence of malaria, which has existed there for a prolonged period, and is believed by some to have had an important bearing upon the fates of the ancient civilisations of the Mediterranean basin. The regions in Europe affected, or seriously affected, by malaria are diminishing yearly. This is now due to conscious efforts, but a similar process has been going on probably for a long period, for many obscure diseases, notably "ague," seem to have been forms of malaria. Their disappearance seems to be due to drainage, which diminishes the breeding places of the mosquitoes, and also to the progress of agriculture, for ponds which form on rich, well-manured land are apparently unsuited to mosquito larvæ. The subject is of great geographical importance, for the spread of man over the surface of the globe, and the progress of civilisation must have been influenced in all time by the prevalence of fly-borne disease. Such diseases have hitherto been the greatest obstacle in the way of the civilisation of Africa. In Uganda extensive tracts of fertile wooded land have had to be abandoned on account of the presence there of the tsetse fly, while, prior to this abandonment, there were districts in which every living soul had been destroyed by the deadly sleeping sickness transmitted by this fly. We can hardly suppose that such facts are without a parallel in human history; and man's distribution over the surface of the globe, and in detail the distribution of his settlements within a country, have doubtless been greatly influenced by the distribution even of such insignificant creatures as the various kinds of flies. Even apart from their power of transmitting disease, the blood-sucking flies must have influenced man in his choice of localities for settlements, and must have been an important factor in the process of adjustment to his surroundings. The naturalist Brehm gives an appalling picture of the number and blood-thirstiness of the mosquitoes of the Siberian tundra, which render life almost intolerable there for both man and beast in summer. Even within the British Islands the uncultivated and undrained regions are often badly infested with small blood-sucking flies, and their numbers must have been vastly greater in the old days before drainage and intensive cultivation had reduced them. It is quite possible that some of the anomalies in regard to the spread of particular races of men over the surface can be explained by the varying susceptibility of different races to insect attack, and there can be no doubt that the blood-sucking insects must have had some effect in determining the rapidity or slowness with which particular tracts were colonised by man.