Fig. 50.—Skeleton of Scelidotherium. (After Capellini.)
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.
In the year 1882 reports were published of the discovery of large footprints—supposed to be human—in a certain sandstone near Carson, Nevada, U.S. The locality was the yard of the State prison, and the tracks were uncovered in quarrying stone for building purposes. Many different kinds of tracks were found, some of which were made by an animal allied to the elephant; some resembled those of the horse and deer; others seem to have been made by a wolf, and yet others by large birds. Those supposed to have been made by human giants were in six series, each with alternate right and left tracks. The stride is from two and a half to over three feet, and each footprint is about eighteen inches long. Now, those who believed these tracks to be human must have found it hard to explain how a giant with a foot some eighteen inches long had a stride no longer than that of an ordinary man of to-day, to say nothing of the fact that the straddle was eighteen to nineteen inches! For these and other reasons Professor Marsh has exploded the idea of their having been made by men, and gave good reasons to show that they were probably made by a giant sloth, such as the Mylodon above mentioned, the remains of which have been discovered in the same strata. They agree in size, in stride, and in width between the right and left impressions, very closely with the tracks that a Mylodon would have made, and it seems that those of the fore feet were afterwards impressed by the hind feet, so that each track contains two impressions.
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.
A few words about these modern sloths may not be out of place; for we shall better understand how they have succeeded in the struggle for existence when we know something of their manner of life; and in some ways they still resemble their great ancestors.
There are few animals which exhibit in a greater degree what appears to the careless observer to be deformity than the sloth, and none that have, on this account, been more maligned by naturalists. Buffon, and many of the older zoologists, were eloquent upon the supposed defects of the unfortunate sloth. These writers gravely asserted that when the sloth ascends a tree, for the purpose of feeding upon its leaves, it is so lazy that it will not quit its station until every trace of verdure is devoured. Some of them even went so far as to assert that when the sloth was compelled, after thus stripping a tree, to look out for a fresh supply of food, it would not take the trouble to descend the tree, but just allowed itself to drop from a branch to the ground. Even Cuvier, who ought to have known better, echoes this tale, and insinuates that Nature, becoming weary of perfection, “wished to amuse herself by producing something imperfect and grotesque,” when the sloths were formed; and he proceeds, with great gravity, to show the “inconvenience of organisation,” which, in his opinion, rendered the sloths unfit for the enjoyment of life.
It is quite true that, on the ground, these animals are about the most awkward creatures that can well be imagined. Their fore legs are much longer than their hind ones; all their toes are terminated by very long curved claws, and the general structure of the animal is such as to prevent them from walking in the manner of an ordinary quadruped, for they are compelled to rest on the sides of their hands and feet. Thus they appear the most helpless of animals, and their only means of progression consists in hooking their claws to some inequality in the ground, and thus dragging their bodies painfully along. But in their natural home, amongst the branches of trees, all these seeming disadvantages vanish—nay, the very peculiarities of structure which render the sloths objects of pity on the ground, are found to render them admirably adapted to their peculiar mode of life. The sloth is a small animal, rarely more than two feet in length, and covered with woolly hair—probably a protection against snakes, its only enemies. It spends nearly the whole of its life in the trees. There, safe from the prowling animals on the ground below, it hangs like a hammock from the bough, and even travels along the branches with its body downwards, using its long claws like grappling-irons.