432 LIFE : OUTLINES OF GENERAL BIOLOGY 



raw materials are furnished by the blood, and probably from 

 tyrosin and related substances. 



In albinos there is obviously an absence of pigmen t -format ion ; 

 and this seems to be due to the loss of an hereditary "factor" con- 

 cerned in the production of the pigment. The pink eyes of true 

 albinos, familiar in white rats and white rabbits, are due to the 

 red blood shining through the unpigmented iris that surrounds 

 the pupil. Of all mammals the Cape Golden Mole has the strangest 

 colouring, for it shows a brilliant metallic lustre, varying from 

 golden-bronze to green and violet of different shades. The precise 

 explanation of this display is unknown, but it must depend on the 

 physical structure of the hairs, not on special pigments. It is an 

 iridescence, changing as we move the animal about. Another 

 antiquity of an animal, the rare Marsupial mole or mole-like Mar- 

 supial of Australia, has an unusual red colour which fades or 

 disappears in museum specimens. As to the green colour of the 

 shaggy hair of the tree-sloths of South and Central America, it is 

 entirely an external addition, being due to the presence of minute 

 green algie, similar to those that paint the trunks of trees in damp 

 situations. They possibly help to make the sloth almost invisible; 

 but it is more probably a very fortuitous association. 



Hrdlicka has made an elaborate study of the colour of the hair 

 in "Old Americans", that is to say those who have not been mixed 

 up during the last three generations with recent immigrants; and 

 uses the following classification. He distinguishes, to begin with, 

 emphatically "black", decidedly "light", and unmistakably "red". 

 That leaves between "blacks" and "lights" three intermediate 

 grades — namely, light brown (not blond), medium brown, and dark 

 brown. A noteworthy fact, familiar to all, is that the colour of 

 the hair, unless it be very dark, changes with age. The flaxen- 

 haired baby becomes a brown-locked boy, and he in turn may 

 become dark, though never black. This means a progressive increase 

 in the production of melanin (until grey hairs begin), but why this 

 should come about no one knows. Another general fact is that in 

 Great Britain and in North America there is a greater proportion 

 of (lark shades among women than among men — perhaps an illus- 

 tration of the general proposition that females are more conser- 

 vative of ancestral conditions. Early PaliEolithic man, who used 

 only rough stone implements, is believed to have had reddish- 

 brown to black hair. 



The appearance of grey hairs before old age is very common, 

 but in very diverse degrees. The tendency is slightly greater among 

 men than among women, (ireyness may mean that in a fresh 

 growth of hair there has been little or no deposition of pigment. 

 Or, as Metchnikoff has shown, an individual hair may turn white 

 Ixcause wjmdering amuboid cells ("chromophages") engulf pigment- 



