THE COLOURS OF ANIMALS AND PLANTS 125 



To fully appreciate the protective value of colours 

 it is necessary to see the animals in their native 

 haunts. Thus, speaking of the Zebra, Francis Galton 

 says that although " no more conspicuous animal can 

 well be conceived," yet the proportion of the black 

 and white stripes " is such as exactly to match the 

 pale tint which arid ground possesses when seen by 

 moonlight." With regard to the Giraffe, Sir S. 

 Baker graphically describes the way in which, when 

 seen at a distance, it resembles a dead tree stem. 

 Again, the green colour of the Sloth is due to parasitic 

 algae, which cause it to resemble a lichen-covered 

 branch ; an oval buff-coloured mark on the back 

 giving the impression of the broken end of the stump. 

 Birds. — The summer plumage of the Ptarmigan 

 conceals it among the heather very effectively ; 

 in winter it becomes white. The Heron again, 

 is almost impossible to find among the rushes, where 

 it stands in an absolutely vertical position, with the 

 tip of its beak tilted up. The ventral surface having 

 pale yellow stripes, closely resembles the surrounding 

 rushes. Moreover, it turns slowly round so as always 

 to present the ventral surface to view, while the 

 striped back and broad dark-coloured sides are never 

 presented to the observer. In the case of birds 

 which build open nests, the female is protectively 

 coloured : this is well seen in the pheasant. When 

 both sexes are brilliantly coloured, such as the king- 

 fisher, parrots, &c, the nest is of such a nature as to 

 conceal the sitting bird. 



Insects.— These afford the best examples of pro- 

 tective colouring both in the larval and adult states. 



