224 



KNOWLEDGE. 



[October, 1901. 



either salt or fresh — comparatively few deriving their 

 nourishment from the moisture contained in tlie air. 

 Some, indeed, are confined to particular descriptions 

 of rock, and possess structui-es recalling roots, but even 

 in these cases it is doubtful if they draw more than 

 an insignificant fraction of their nutriment from the 

 substance on which they grpw. 



In the moist tropical forests forming the home of the 

 sloths the algae in the cracks of their hairs glow 

 readily, and thus communicate to the entire coat that 

 general green tint, which, as already said, is reported 

 to render them almost ' indistinguishable from the 

 clusters of lichen among which they hang suspended. 



" In thick transverse sections of the hair," writes 

 Dr. Ridewood, " these algal bodies show up very clearly, 

 since they stain deeply, and have a sharply defined 

 circular or slightly oval outline. Unless the hair is 

 much broken, they are confined to the outer parts of 

 the extra^cortical layer." 



Not the least curious phase of a marvellous subject 

 is that the two-t«ed slotb, although the structure of its 

 hair is very different from that of the ai, also has an 

 alga, which belongs to a species Cjuite distinct from the 

 one found in the former. 



In the two-toed sloth the hairs lack the outer sheath 

 investing those of the ai, and consist chiefly of the 

 central core of coi-tex; in other words, they conespond 

 to those hairs of the latter from which the outer sheath 

 lias been shed. The surface of these hairs is dis- 

 tinctly furrowed with longitudinal grooves or 

 channels, and it is in these channels that the alga 

 distinctive of this particidar species is lodged and 

 flourishes. After stating that a solution capable of 

 exhibiting the absorption bauds of the vegetable 

 colouring matter chlorophyll can be obtained from the 

 hairs of this animal. Dr. Ridewood gives the following 

 particulars with regard to their structure : — 



" The hairs are, as a rule, coaise, and with a single 

 curve extending over the greater part of the length, 

 while the basal fourth or so is wavy; but in young 

 specimens, and in some apparently adult examples from 

 Costa Rica, the hair is vei-y delicate and soft, and 

 siNuous from base to point. However, in these forms 

 the hairs .... have only two or three furrows 

 instead of the more usual nine, ten, or eleven. The 

 algre, also, arc quite absent from many of the gi-ooves. 

 When such an empty groove is examined in optical 

 section ■ it exhibits the outlines of obsolete extras 



cortical cells In baby specimens more than 



half of the hairs ar-e slender non-medullate cylinders, 

 with a very distinct scaly cuticle, and no grooves on 

 the surface." 



These simple hairs are, in fact, the only rudiments 

 of an under-fur possessed by the two-toed sloth, or 

 unau. 



• It may be added that in the presumably extinct 

 ground-sloths (the skin of one of which has fortunately 

 been presei-ved to us in a cave in Patagonia) the hairs 

 are solid, without (according to Dr. Ridewood) any 

 trace of the outer sheath of those of the ai, or of the 

 flutings characterising those of the unau. These ai-e 

 thus evidently of a less specialised type than is the 

 haii7 covering of the modern tree sloths, as indeed 

 would naturally be expected to be the case in the 

 members of the ancestral group to whicii the latter 

 ]n-obably trace their descent. 



The above, then, aiv the essential facts with reirard 



to the peculiarities of their hair by means of which the 

 sloths are brought into such special and remarkable 

 hamiony with their environment, and it now remains 

 to consider how best to explain their origin. 



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 

 cei-tain 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 types of sloth were 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 convenient 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^produciug 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 

 inherited, their progeny would consequently have the 

 best chance of winning in life's race. It is, of coiu^se, 

 not difficult to assume that when the alga had once 

 become firmly established as pai't 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 ai-ises one of the many difficulties connected 

 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 

 ai-boreal kind of life, and sisce 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 sepai-ate occasions an alga 

 has independently taken advantage of this suitable 

 vacant situation and adapted itself to its new surround- 

 ings. This difficulty, like the one connected with sloths 

 having flourished before they accjuired a lichen-gi'owth, 

 may appear of little importance to those who are con- 

 vinced 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 peculiai', and has therefore doubtless some 

 special purpose. And, to put it shortly, the question 

 consequently is whether these two tyj^es of hair 

 structure were specially developed for the reception and 

 growth of algre designed to aid in the protection of the 

 animals on which they occur, or whether such develop- 

 ment has taken place for some totally different object, 

 and that the subsequent growth of the algie, 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 peculiai- character is largely protective, and it 

 would be a most cvn-ious coincidence had this protective 



