EMBRYOLOGY. 137 
organs, the abnormal in one animal being normal in 
another. Occasionally we find animals so badly organized 
as to make it incredible that they should have appeared on 
the earth as such originally. «In speaking of the Sloth, 
Cuvier observes, “One finds them so little related to ordi- 
nary animals, the general laws of living organizations apply 
so little to them, the different parts of their body seem to 
be so much in contradiction with the rules of co-existence 
that we find established in the whole animal kingdom, that 
one could really believe that they are the remains of 
another order of things.” Cuvier then continues by saying 
that in most forms the disadvantages are compensated by 
advantages, “but in the Sloth each singularity of organiza- 
tion seems to result only in feebleness and imperfection, 
and the inconveniences belonging to- the animal are not 
compensated by any advantage.”* 
The only explanation, at present, of the existence of such 
a wretched animal as the Sloth is that it is the degenerated 
representative of some extinct animal who lived at the 
same time as the Megatherium, which it resembles in the 
form of its head. The limbs and backbone of the Mega- 
therium are, however, represented by the Great Ant-eater. 
The Sloths and Great Ant-eater are confined to South 
America, and it is there that the Megatherium remains 
have been found in great abundance. 
The development of the flower through the gradual 
metamorphosis of the leaf is a beautiful illustration of the 
evolution of different forms from a common type. In the 
words of Prof. Gray, * The leaves of the stem, the leaves 
or petals of the flower, and even the stamens and pistils, 
are all forms of a common type, only differing in their 
special development; and it may be added that, in an 
early stage of development, they all appear nearly alike. 
* Cuvier, * Ossemens fossiles." 
