When dealing with the changes, cataclysmic or gradual, which divide one period of palæontological history from another, we can sometimes assign causes, and again we cannot even guess at them. In the case of single species, or of faunas of very restricted localities, the explanation is often self-evident. A comparatively slight change in the amount of moisture in the climate, with the attendant change in vegetation, might readily mean the destruction of a group of huge herbivores with a bodily size such that they needed a vast quantity of food, and with teeth so weak or so peculiar that but one or two kinds of plants could furnish this food. Again, we now know that the most deadly foes of the higher forms of life are various lower forms of life, such as insects, or microscopic creatures conveyed into the blood by insects. There are districts in South America where many large animals, wild and domestic, cannot live because of the presence either of certain ticks or of certain baleful flies. In Africa there is a terrible genus of poison fly, each species acting as the host of microscopic creatures which are deadly to certain of the higher vertebrates. One of these species, though harmless to man, is fatal to all domestic animals, and this although harmless to the closely-related wild kinsfolk of these animals. Another is fatal to man himself, being the cause of the "sleeping sickness" which in many large districts has killed out the entire population. Of course the development or the extension of the range of any such insects, and any one of many other causes which we see actually at work around us, would readily account for the destruction of some given species or even for the destruction of several species in a limited area of country. When whole faunal groups die out over large areas, the question is different, and may or may not be susceptible of explanation with the knowledge we actually possess. In the old arctogæal continent, for instance, in what is now Europe, Asia, and North America, the glacial period made a complete, but of course explicable, change in the faunal life of the region. At one time the continent held a rich and varied fauna. Then a period of great cold supervened, and a different fauna succeeded the first. The explanation of the change is obvious.