CHAP, vii The Industries of Animals 123 



paper houses which wasps construct, are well known ; but we 

 should not forget the architecture of the mason-bees, the 

 great towers of the termites, and the lodges of the beavers. 



Perhaps I may be allowed to notice once again, what I 

 have suggested in another chapter, that while many of the 

 shelters which animals make are for the young rather than 

 for the adults, the line of definition is not strict, and some 

 which were nests to "begin with have expanded into homes 

 an instance of a kind of evolution which is recognisable 

 in many other cases. 



5. Movements, But animals are active in other ways. 

 All their ways of moving should be considered the marvel- 



FIG. 28. Flight of crested heron, ten images per second. (From Chambers's 

 Encyclop, ; after Marey.) 



lous flight of birds and insects, the power of swimming 

 and diving, the strange motion of serpents, the leap, the 

 heavy tread, the swift gallop of Mammals. All their 

 gambolings and playful frolics, their travels in search of 

 food, and their migrations over land and sea, should be 

 reckoned up. 



Most marvellous is the winged flight of birds. As a 

 boat is borne along when the wind fills the sails, or when 

 the oars strike the water, and as a swimmer beats the 

 water with his hands, so the bird beating the air backwards 

 with its wings is borne onward in swift flight. But the 

 air is not so resistent as the water, and no bird can float in 

 the air as a boat floats in the water. Thus the stroke has 



