320 MOSTLY MAMMALS 
with this sort of explanation. It is quite clear that an 
alga would have been of no advantage to the sloths until 
they had acquired their present completely arboreal kind 
of life, and since there is a considerable probability that 
both types of these animals were independently derived 
from some of the smaller ground-sloths, it follows that on 
two separate occasions an alga has independently taken 
advantage of this suitable vacant situation and adapted 
itself to its new surroundings. This difficulty, like the 
one connected with sloths having flourished before they 
acquired a lichen-growth, may appear of little importance 
to those who are convinced of the all-sufficiency of natural 
selection, but to others it may (if well founded) seem more 
serious. 
As we have already seen, the structure of the hair in 
the two types of sloth is, each in its own way, absolutely 
peculiar, and has therefore doubtless some special purpose. 
And, to put it shortly, the question consequently is whether 
these two types of hair structure were specially developed 
for the reception and growth of algas designed to aid in 
the protection of the animals in which they occur, or whether 
such development has taken place for some totally different 
object, and that the subsequent growth of the algas, and 
the additional protection thereby afforded, have been purely 
fortuitous. The fact that the hairs themselves assimilate 
the body of the sloth to a lichen-clad knot shows that 
their peculiar character is largely protective, and it would 
be a most curious coincidence had this protective resemblance 
been enhanced by an accidental growth of algas. 
As regards the manner in which the growth of algas is 
maintained in the sloths from one generation to another, 
the only rational explanation which presents itself is that 
the young sloths become infected with alga-spores from 
