534 THE WONDER OF LIFE 



adaptations may have been wrought out, this does not 

 lessen the wonder of that variability that supplies the raw 

 material. 



Colour Adaptations. There is great wealth of colouring 

 in the animal kingdom. Humming-birds, tropical fishes, 

 mollusc-shells, butterflies, starfishes, and sea-anemones 

 immediately occur to one, and it would be easy to mention 

 a hundred gorgeous examples. The colour is partly due to 

 pigmentary substances made by the animals ; partly to 

 the physical structure of the surface e.g. the occurrence 

 of thin lamellae or very delicate sculpturings which cause 

 interference of light ; and partly to a combination of pig- 

 ment and some peculiar physical structure. The redness 

 of the blood is due to a pigment haemoglobin ; the 

 iridescence of many a shell is wholly due to the physical 

 structure; the coloration of a peacock's feather is due 

 to a combination of the two kinds. 



It may be noted, for the sake of completeness, that 

 some animals owe their colour to other organisms which 

 live in association or partnership with them. Thus the 

 common green Hydra is green because of minute partner 

 Algae which live within its transparent cells. And at the 

 other end of the scale we find that the shaggy S. American 

 tree-sloth (Bradypus) is greenish, because of minute Algae 

 that, strangely enough, find a living on its rough hair. 



In the simple marine worm, Convoluta roscoffensis, so 

 carefully studied by Professors Keeble and Gamble, the 

 green colour is due to a unicellular Alga which lives in part- 

 nership with the cells of the worm's body. The newly - 

 hatched worm is colourless, and has to be infected from the 

 sea- water or from the egg-capsules on which the Alga habitu- 

 ally settles. A curious point is that the green cells taken 



