338 DE. H. C. SORBY ON THE GREEN COLOUR 



the entire skin at my disposal. I have also had the advantage of 

 studying the hair of specimens which had lived some years in 

 England; and was thus able to learn that none of the green algae 

 grew on the hair in the dry atmosphere of the house in the 

 Gardens, whereas on that of the animal fresh from its native 

 damp woods the number on the hair from some parts of the skin 

 was so great as to give rise to a most unmistakable green colour, 

 not seen in the case of the animals which have lived some time 

 in this country. It therefore appears that the growth of the 

 alga depends partly on the damp character of the locality in 

 which the Sloths live. I, however, think that it also depends, in 

 great measure, on the most exceptional and remarkable structure 

 of the hair of Sloths ; and after having carefully studied that of 

 very many other animals, I must say that it appears to me not 

 at all probable that algae would grow on the hair of other mam- 

 mals, even in damp localities. 



For my present purpose it is convenient to look upon hair as 

 a very variable mixture of dense horn and a highly cellular 

 pithy substance, containing much air. As an example of one 

 extreme, I may refer to the bristles of the wild boar, which are 

 generally almost exclusively composed of dense horn. In many 

 animals the hair consists of a solid external sheath of horn, 

 with a central pithy core, the relative size of which varies much ; 

 and in the case of nearly all Deer and some Antelopes and allied 

 animals this central pith constitutes nearly the whole, and 

 the external horny layer is very thin. In all these cases this 

 layer is continuous over the whole exterior ; and though some- 

 times the surface is rough and scaly, yet in many cases it is 

 almost or quite smooth and glossy. 



The hair of Cholopus (figs. 4 & 5) differs in a most remarkable 

 manner from all others that I have examined. Instead of the horny 

 exterior being continuous, it is more or less deeply fluted longi- 

 tudinally, right down to the central pith, which is thus exposed 

 along the bottom of the numerous grooves (fig. 4). The growth 

 of the green algae is most unmistakably related to this structure. 

 None grow on the surface of the bright glossy ribs, whereas all 

 along the depressions they abound, so that we see clear polished 

 ribs and deep green hollows extending longitudinally along each 

 hair (fig. 5). I do not see how we can doubt that this special 

 localization is due to the fact that the surface of the grooves is 

 rougher, as well as more protected from friction, than the ribs. 



