The reader who has some knowledge of natural history will not need to be told that the sloths of the present day, inhabiting the same region as their gigantic ancestors, are of small size, and live among the branches of the trees, together with the spider monkeys, howlers, and other apes. An interesting question to the evolutionist is--How did the change take place from the old huge and heavy types to the smaller and agile types of the present day? Can it be possible that the more difficult and tedious task of pulling down branches and even stems of trees, in order to devour the leaves, was abandoned for the simpler method of climbing up and feeding among the branches? It certainly looks as if a change of this kind had been instituted at some distant period in the past--distant, that is, to _us_, but not very remote geologically. The present method seems so much simpler that we need not be surprised at its adoption, for Nature is ever ready to encourage and assist those among the children of Life which can hit upon and adopt new and improved methods, either in obtaining food or repelling enemies, or other duties imposed upon them. Now, suppose that, in accordance with the well-known fact that variations in the offspring of animals are constantly cropping up, some considerably smaller variety of Megatherium, or Mylodon, or other now extinct type, appeared on the scene, and, by virtue of its comparative agility, could climb a tree and feed among the branches instead of pulling them down: then, as Darwin has so well explained, Nature would seize upon this accidental variation, and give it an advantage over its more awkward relations. Its offspring, too, would inherit the same characteristics, they would adopt the same habits, and, in time, as "natural selection" further increased these characters, by weeding out those that were unfit while fostering all those that were neither large nor clumsy in climbing trees, a new race of sloths would arise. This new race, it can well be imagined, would in time outstrip the old race in numbers, for successful races multiply while unsuccessful ones diminish. Victory is not always to the great and the strong, for cunning and quickness are often of more service than mere brute strength; and perhaps the sloths, as we now see them in the Brazilian forests, have hit upon "a new and original plan" by means of which the old colossal forms described above have been driven out of the field, and so exterminated by a process of competition. Such an explanation would be in thorough harmony with modern teaching, and, as the other suggestion about long-continued droughts, given on p. 184, may not appear satisfactory to some of our readers, we offer this theory for what it may be worth.