3i8 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 a'i, 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. 



