October, 1901.] 



KNOWLEDGE 





They were grivct monkoys.|i of a beautiful greeuish-groy 

 on I lie back, with whitisli bi"cast#, and bhiok faces 

 adorned with handsome white whiskers. We uotiied 

 tliat "they were very fond of tJie gum which exuded from 

 the acacia trees. But mammals of any kind were scarce 

 iu the couutiy wo worked, and we did not travel far 

 enough south for big game. Occasionally a gazelle 

 or an ;uitelopo woidd be seen, but they were rai'o and 

 shy. Ill two or tlu'ee places there were hares, and it 

 was while I was chasing a wounded hare through some 

 rough grjiss that I came across the only bird 

 new to science which we discovered. This was a 

 tiny fantail warbler.§§ of a most delicate light sandy 

 coloiU' on all the upper paj-ts and with a silveiy breast — 

 a colouring most suitable for the brown grass and sandy 

 countn- which it inhabit<.»d. I only obtained one speci- 

 men, and It is somewhat risky to found a new species on 

 a single example, but the Hon. Charles Rothschild 

 has lately obtained an exactly similar bird near Shendy. 

 so that the claims of my new species may now be con- 

 sidered as established. 



The photogi-aphs, with which this seiies of ai-ticles is 

 illusti'ated, were taken by Mr. C. F. Camburu, to whom 

 I am much indebted for permission, to make use of them. 



PLANT-BEARING HAIR. 



By R. Lydekker. 



Although the name " sloth ' is not unfrequently mis- 

 applied by travellers to the slow lemurs of India and 

 the Malay countries, or to their cousins the galagos of 

 Africa, it should, as many of my readci-s are doubtless 

 aware, be restricted to certain peculiar mammals 

 inhabiting the tropical forests of Central and South 

 America. In addition to the simple (Character of their 

 teeth, which are confined to the sides of the jaws, sloths 

 are chai-acterised by their short faces, rudimentary tails, 

 shaggy coats, and hook-like claws by means of which 

 they hang suspended, back-downwai'ds. from the 

 branches of the trees among which their lives are spent. 

 Two very distinct tyjjes of these animals are known, 

 readily distinguished by the number of toes on the fore- 

 limb. In the one form — the three-toed sloth — there 

 are three claws on each foot, both in the front and the 

 hind limbs. But iu the other — the two-toed sloth — 

 there are only two claws on each of the fore-feet. 



These, however, are by no means the only differences 

 between the two types (and I say types rather than 

 species, because ifc is quit© probable that each 

 modification has more than a single specific represen- 

 tative). In the first place, there is a difference in the 

 form and position of the first tooth in each jaw. In 

 the three-toed sloth, or ai, for instance, this tooth is 

 similar in form to those behind it, from the first of 

 which it is separated by a space not longer than the 

 one between the second and third. In the two-toed 

 form, on the other hand, the fii-st tooth is taller than 

 those behind, and has a bevelled, instead of a flat 

 gi-inding surface, while the space dividing it from the 

 second much exceeds that between any of the others. 

 Again, the front of the upper jaw of the two-toed sloth 

 carries a T-shaped bone, corresponding to the pre- 

 maxillae of other mammals, which is totally wanting in 

 the other form. The front of the lower jaw of the 

 former is also prolonged so as to form a kind of spout, 

 of which there is no trace in the latter. In both 

 these respects the two-toed sloth comes much nearer to 



XX Cercopithecu3 aabaeiu (Jjian.). §§ CiXicofo aWt^aZo, Witherby. 



the extinct ground-sloths (oi which we have lately 

 heard so mucii) than is the ca.sc with its three-clawed 

 cousin. 



Again, if the males of the three-toed sloth be 

 examined, there will be seen a patch in the middle of 

 the back where, owing to the absence of the long coarse 

 ext<?rnal hair, the presence of a soft orange and brown 

 under-fur is shown. It has been stated that this patch 

 of under-fur is made visible by the animals rubbing 

 their backs against boughs and wearing off the long 

 hair, but it seems much more probable that it is a 

 sexual character. Of this uuder-fur the two-toed sloth 

 has but a very imperfect development. 



Apart from its extremely coarse and brittle nalurc, 

 the most striking peculiarity of the outer hair of the 

 sloths is its more or less decidedly gi-een tinge. To see 

 this in perfection it is neccssar-y to examine living 

 animals, as it tends to fade away more or less com- 

 pletely in skins which have been long exposed to the 

 light, leaving the hair of a p;de greyish brown coloiu-. 

 The green tint is, however, still distinctly visible in a 

 pair of mounted specimens displayed to the public in 

 the Natural History Branch of the British Museum. 



Now green is a. vciy rare colour among mammals, 

 and there ought therefore to bo some special rea.son for 

 its development in the sloths. And, as a matter of 

 fact, the means by which this coloration is produccfl 

 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 typo of coloration is, of course, 

 to assimilato 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 i-ender the sloth almost indistinguishable from the 

 bunches of grey-gi-ccn lichens among which it dwells. 

 And if the physical means by which this gi-een tinge in 

 the hair of the sloths is produced bo little short of 

 marvellous, what is to be said with regard to the 

 inducing cause of the phenomenon? But of this anou. 



If a few hairs of the a'i 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 tochnically known as cortex 

 (and not of the medulla which fomis the core of the 

 hair of many mammals), the outer sheath is composed 

 of an altogether peculiar stnicture, for w'hich the some- 

 what cumbersome name of extra-cortex has been 

 proposed. Possibly it may correspond to the thin 

 cuticle of more ordinaiy hairs ; possibly not. — either way, 

 it need not concern us further on this occasion. In 

 old and wont hairs this outer sheath (as it will bo more 

 convenient to call it) becomes brittle and breaks away 

 piecemeal, leaviirg 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.* And for the benefit of non-botanical readers 

 it may bo well to mention here that algae (among which 

 sea-weeds aro included) form a group of flowerless plants 

 related on the one hand to the funguses and on the 

 other to the lichens. The majoz-ity live in wator — 



* Tlif iircscnc'c of alga; iu the hair of slotli.s iias Ijueil luii^ knoHii 

 to science, but the whole sul)ject has reeeutlv been re-inrestigaled hy 

 l)i'. W. G. Bidovvood {Quart. Journ. Microscopical Science), and it 

 is the appearauec of his coiumuuicatiou that has suggested the i>resent 

 article. 



