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 ai 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 
