SLOTHS AND THEIR HAIR 319 



Of all the problems with which the naturalist has to 

 deal, those connected with the "mimicry" of one animal 

 by another, or the special resemblances by certain animals to 

 their inanimate surroundings, are some of the most difficult, 

 and the present instance forms no exception to this rule, if 

 it is believed that " natural selection," or some such mode 

 of evolution, has been the sole factor in the case. 



In this instance, at any rate, there can be no question as 

 to any volition on the part of the animal concerned having 

 aided in the development of its protective resemblance. 

 And, on the hypothesis of natural selection, it appears 

 necessary to assume that when the modern type of sloths 

 was first evolved no alga grew in the hair of these animals, 

 which were consequently able to exist and flourish without 

 any such adventitious aid. The nature of their hair formed, 

 however, in the case of each of the two groups, a con- 

 venient nidus for the lodgment and growth of an alga ; 

 and such a suitable situation was accordingly in each 

 instance seized on as a habitat by one of those lowly 

 plants. At first, of course, only a certain number of 

 sloths would have had alga-producing hair, and these, 

 from the green tinge of their coats, would consequently 

 enjoy a better chance of escape from foes than would their 

 brethren which had not yet acquired the greenish garb. 

 And, on the assumption that alga-growing hair is in- 

 herited, their progeny would consequently have the best 

 chance of winning in life's race. It is, of course, not 

 difficult to assume that when the alga had once become 

 firmly established as part and parcel of the hair of each 

 group it acquired in both cases distinct specific characters, 

 even if there were not originally two kinds of these plants 

 concerned. 



And here arises one of the many difficulties connected 



