144 FOSSIL MAMMALIA. 
in harmonious subordination to this chief purpose 
in the animal economy. 
In the case of ordinary animals, the passage 
from one form to another is so gradual, and the 
functions of one species receive such ample and 
obvious illustrations from those of the species 
adjacent to it, that we are rarely at a loss, to see 
the final cause of almost every arrangement that 
is presented to the anatomist. This is more es- 
pecially the case with respect to the skeleton, 
which forms the foundation of all the other me- 
chanisms within the body, and is of the highest 
importance in the history of fossil animals, of 
which we rarely find any other remains besides 
the bones, and teeth, and the scaly or osseous in- 
teguments. I select the Megatherium, because 
it affords an example of most extraordinary 
deviations, and of egregious apparent monstro- 
sity ; viz. the case of a gigantic animal exceed- 
ing the largest Rhinoceros in bulk, and to which 
the nearest approximations that occur in the 
living world, are found in the not less anoma- 
lous genera of Sloth, Armadillo, and Chlamy- 
phorus ; the former adapted to the peculiar 
habit of residing upon trees ; the two latter 
constructed with unusual adaptations to the 
habit of burrowing in search of their food and 
shelter in sand ; and all limited in their geogra- 
phical distribution, nearly to the same regions 
of America that were once the residence of the 
Megatherium. 
