316 MOSTLY MAMMALS 



means by which this coloration is produced is one of the 

 most marvellous phenomena in the whole animal kingdom 

 so marvellous, indeed, that it is at first almost impossible 

 to believe that it is true. The object of this peculiar type 

 of coloration is, of course, to assimilate the animal to its 

 leafy surroundings and thus to render it as inconspicuous 

 as possible ; and when hanging in its usual position from 

 the under-side of a bough, its long, coarse, and green-tinged 

 hair is stated to render the sloth almost indistinguishable 

 from the bunches of grey-green lichens among which it 

 dwells. And if the physical means by which this green 

 tinge in the hair of the sloths is produced be little short 

 of marvellous, what is to be said with regard to the inducing 

 cause of the phenomenon ? But of this anon. 



If a few hairs of the a'i be examined under the microscope 

 by a person familiar with the structure of hair in general, 

 it will be found that while the central portion consists of 

 what is technically known as cortex (and not of the medulla 

 which forms the core of the hair of many mammals), the outer 

 sheath is composed of an altogether peculiar structure, for 

 which the somewhat cumbersome name of extra-cortex has 

 been proposed. Possibly it may correspond to the thin 

 cuticle of more ordinary hairs, possibly not ; either way, it 

 need not concern us further on this occasion. In old and 

 worn hairs this outer sheath (as it will be more convenient 

 to call it) becomes brittle and breaks away piecemeal, leaving 

 the central core alone. 



But in ordinary circumstances the sheath tends to form 

 a number of transverse cracks, and in these cracks grows 

 a primitive type of plant namely, a one-celled alga. For 

 the benefit of my non-botanical readers it may be well to 

 mention here that algas (among which sea-weeds are in- 

 cluded) form a group of flowerless plants related on the 



