96 



ANIMAL COLOKATION. 



sluggishness of the animal, this colour may have a protective 

 value. The colour, however, curiously enough, is purely ad- 

 ventitious, and is the only example of the kind among the 

 Mammalia. 



In captivity the sloth gradually loses this green colour, and 

 becomes greyer ; this is not a change comparable to the 

 gradual fading of the brilliant red of the scarlet Ibis, but is 

 simply due to the disappearance of certain minute Alga?, 



Fig. 5.— The Sloth. 



which, as Mr. Sorby first discovered, are the cause of the 

 peculiar coloration ; doubtless it is the absence of a damp 

 and warm climate that is fatal to the Algas, and is the cause, 

 therefore, of what appears to be an actual change in the colour 

 of the hair itself. It would be difficult, therefore, to set down 

 this case of what appears to be an adaptive coloration to the 

 action of natural selection ; though it must be admitted that 

 we still have to explain why this animal alone should be 

 subject to the intrusion of the Algte : possibly its sluggish 



