16 ANIMAL COLORATION. 



other tree-frequenting birds, but this colour does not occar 

 in plenty of other genera and families which equally live 

 among trees. 



Green appears to be an impossible colour among rodents and 

 marsupials, and, indeed, among mammals in general, to which 

 rule the "green" Cercopithecus and the sloth* are hardly 

 exceptions. The Echinoderms as a group are distinguished by 

 shades of a brown or red to a purple coloration ; but the 

 wide distribution of certain colours is, perhaps, most strikingly 

 shown in the Mammalia. 



Among butterflies we meet with the same thing. The 

 Lycsenidfe (" Blues ") are generally, as their name denotes, 

 blue ; but they are also chai-acterised by the eye-spots on the 

 under surface of the wings. The mottling of the underwings 

 of the Vanessida3 is another example ; so, also, is the " silver- 

 ing " of the underwings of the fritillaries, and the tawny 

 coloration of the upper surface. 



Besides, constancy of colour occurs among animals where it 

 can hardly be of much use. It would be difficult to say in 

 what way one species of earthworm is profited by having 

 a bluish purple coloration, another in having a decidedly 

 greenish tinge, and a third in being bright red. And yet it 

 is perfectly possible to distinguish species by their coloration, 

 as any one who takes a walk after a rainy night maj' see for 

 himself. The earthworm has enemies above ground as well 

 as below : the colours could obviously not be seen below 

 ground ; and it has yet to be proved that the rook, as he follows 

 the plough, exercises a deliberate choice in the colour of the 

 worms which are selected as food. Moreover, the same colours 

 are met with in earthworms inhabiting different parts of the 

 world, and as unlike in structure as they can be ; this would 

 * See, for the real cause of the green colour of the Sloth, p. 96. 



