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 



