These monster sloths inhabited South America during the latest geological period, known as the Pleistocene. During part of that time North America, as well as Northern Europe and Asia, were invaded by a great ice-sheet, and an arctic climate prevailed. It is therefore very probable that while the mammoth and the mastodon were roaming over North America, giant sloths and armadillos were monarchs of the southern continent. What cause, or causes, led to the extermination of the giant sloths and armadillos is still a matter of speculation. One writer suggests an explanation that seems to deserve consideration. The southern parts of this great continent are even now subject to long-continued droughts, sometimes lasting for three years in succession, and bringing great destruction to cattle. In fact, the discoveries related above were rendered possible by several successive dry seasons. It is argued that the upright position of most of the skeletons found _in situ_ seems to suggest that the creatures must have been mired in adhesive mud sufficiently firm to uphold the ponderous bones after the flesh had decayed. A long drought would bring the creatures from the drained and parched country to the rivers, reduced by want of rain to slender streams running between extensive mud-banks; and it is possible that, in their anxious efforts to reach the water, they may have only sunk deeper and deeper in the mud until they were engulfed. This idea is strengthened by information supplied to Mr. Darwin when in these parts (recorded in his _Journal_). An eye-witness told him that during the _gran seco_, or great drought, the cattle in herds of thousands rushed into the Parana, and, being exhausted by hunger and thirst, were unable to crawl up the muddy banks, and so were drowned. In the last great drought, from 1830 to 1832, it is probable (according to calculations made) that the number of animals that died was over one million and a half. The borders of all the lakes and streamlets in the province were long afterwards white with their bones.