But whilst it is perfectly evident that the long limbs and their joints, and the peculiar turning in of the ankles, and the structure of the clawed hands and feet, are all admirably adapted for the peculiar life which the animal leads, it appears to be consistent with anatomical reasoning to believe that the Sloth is an instance of retrograde development; that, in fact, the peculiar formation of the skull, neck, wrists, and ankles, is the result of the laws of disuse and adaptation operating on ancestral animals, which once had their anatomy more consistent with a perfect mammalian type. [Illustration: SKELETON OF THE SLOTH.] When the Sloths were first carefully watched and studied, their length of neck and their ability to turn the head, so as to look at a person standing directly behind or beneath them, without swerving the body, struck Mr. Burchell especially. This curious peculiarity led to a careful examination of the skeleton of the different kinds, and much discussion followed, for it was found that in the Sloth examined (the Three-clawed Ai) there were more neck bones (vertebræ of the cervical region) than in other Mammalia. Instead of the common number of seven neck bones, there were nine. This elongation of the neck of course permitted a greater amount of twisting than could occur in an animal with fewer neck bones. But there are other reasons why the head can be so much twisted round, for the spines on the neck bones are small, and the joint between the skull and the first vertebra is so fashioned that this remarkable motion is possible. There was a great deal of discussion about the extra neck bones, and as the last two had rib-like projections from their sides, some anatomists considered them to belong to the true rib-bearing vertebræ, or those of the back (the dorsal). But when the other Sloths were examined it was found that the number of the bones of the neck in all the two-fingered kinds was not as great as in other animals. There are only six neck vertebra in one well-known species (_Cholœpus Hoffmanni_, for instance), whilst there are seven in another two-toed Sloth.