318 MOSTLY MAMMALS 
capable of exhibiting the absorption bands of the vegetable 
colouring-matter chlorophyll can be obtained from the hairs 
of this animal, Dr. Ridewood gives the following particulars 
with regard to their structure :— 
“The hairs are, as a rule, coarse, and with a single curve 
extending over the greater part of the length, while the 
basal fourth or so is wavy; but in young specimens, and 
in some apparently adult examples from Costa Rica, the 
hair is very delicate and soft, and sinuous from base to 
point. However, in these forms the hairs ... have only 
two or three furrows instead of the more usual nine, ten, 
or eleven. The algas, also, are quite absent from many of 
the grooves. When such an empty groove is examined 
in optical section it exhibits the outlines of obsolete extra- 
cortical cells... . In baby specimens more than half of 
the hairs are slender non-medullate cylinders, with a very 
distinct scaly cuticle, and no grooves on the surface.” 
These simple hairs are, in fact, the only rudiments of 
an under-fur possessed by the two-toed sloth, or unau. 
It may be added that in the extinct ground-sloths (the 
skin of one of which has been preserved in a cave in 
Patagonia) the hairs are solid, without any trace of the outer 
sheath of those of the ai, or of the flutings characterising 
those of the unau. These are thus evidently of a less 
specialised type than is the hairy covering of the modern 
tree-sloths, as indeed would naturally be expected to be 
the case in the members of the ancestral group from which 
the latter probably trace their descent. 
The above, then, are the essential facts with regard to 
the peculiarities of their hair by means of which the sloths 
are brought into such special and remarkable harmony with 
their environment, and it now remains to consider how best 
to explain their origin. 
