150 The Outline of Science 



Very beautiful is the way in which many birds, like our com- 

 mon chaffinch, disguise the outside of their nest with moss and 

 lichen and other trifles felted together, so that the cradle is as 

 inconspicuous as possible. There seems to be a touch of art in 

 fastening pieces of spider's web on the outside of a nest! 



How curious is the case of the tree-sloth of South American 

 forests, that walks slowly, back downwards, along the undersides 

 of the branches, hanging on by its long, curved fingers and toes. 

 It is a nocturnal animal, and therefore not in special danger, but 

 when resting during the day it is almost invisible because its 

 shaggy hair is so like certain lichens and other growths on the 

 branches. But the protective resemblance is enhanced by the 

 presence of a green alga, which actually lives on the surface of 

 the sloth's hairs an alga like the one that makes tree-stems and 

 gate-posts green in damp weather. 



There is no commoner sight in the early summer than the 

 cuckoo-spit on the grasses and herbage by the wayside. It is 

 conspicuous and yet it is said to be left severely alone by almost 

 all creatures. In some way it must be a disguise. It is a sort 

 of soap made by the activity of small frog-hoppers while they 

 are still in the wingless larval stage, before they begin to hop. 

 The insect pierces with its sharp mouth-parts the skin of the plant 

 and sucks in sweet sap which by and by overflows over its body. 

 It works its body up and down many times, whipping in air, 

 which mixes with the sugary sap, reminding one of how "whipped 

 egg" is made. But along with the sugary sap and the air, there 

 is a little ferment from the food-canal and a little wax from glands 

 on the skin, and the four things mixed together make a kind of 

 soap which lasts through the heat of the day. 



There are many other modes of disguise besides those which 

 we have been able to illustrate. Indeed, the biggest fact is that 

 there are so many, for it brings us back to the idea that life is not 

 an easy business. It is true, as Walt Whitman says, that animals 

 do not sweat and whine about their condition ; perhaps it is true, 



