Whatever may have happened in past geological epochs, by force of great earth-movements which rapidly brought the adaptations of one continent into contact with the parasites of another, it is quite certain that man, proud man, ever since he has learnt to build a ship, and even before that, when he made up his mind to march aimlessly across continents till he could go no further, has played havoc with himself and all sorts of his fellow-beings by mixing up the products of one area with those of another. Nowhere has man allowed himself--let alone other animals or even plants--to exist in fixed local conditions to which he or they have become adjusted. With ceaseless restlessness he has introduced men and beasts and plants from one land to another. He has constantly migrated with his herds and his horses, from continent to continent. Parasites, in themselves beneficent purifiers of the race, have been thus converted into terrible scourges and the agents of disease. Europeans are decimated by the locally innocuous parasites of Africa; the South Sea islanders are exterminated by the comparatively harmless measles of Europe. A striking example of the disasters brought about by man’s blind dealings with Nature--disasters which can and will hereafter be avoided by the aid of science--is to be found in the history of the insect phylloxera and the vine. In America the vine had become adjusted to the phylloxera larvae, so that when they nibbled its roots the American vine threw out new root-shoots and was none the worse for the little visitor. Man in his blundering way introduced the American vine, and with it the phylloxera, to Europe; and in three years half the vines in France and Italy were destroyed by the phylloxera, because the European vines had not been bred in association with this little pest, and had not acquired the simple adjusting faculty of throwing out new shoots. But it is not only by his reckless mixing up of incompatibles from all parts of the globe that the unscientific man has risked the conversion of paradise into a desert. In his greedy efforts to produce large quantities of animals and plants convenient for his purposes, and in his eagerness to mass and organise his own race for defence and conquest, man has accumulated unnatural swarms of one species in field and ranch and unnatural crowds of his own kind in towns and fortresses. Such undiluted masses of one organism serve as a ready field for the propagation of previously rare and unimportant parasites from individual to individual. Human epidemic diseases as well as those of cattle and crops, are largely due to this unguarded action of the unscientific man.