INCUBATING HEAT 



215 



, its productiveness has to be increased or loss of heat 

 checked. The great voracity of small birds con- 

 tributes to maintain 

 the production of heat, 

 and in order to con- 

 centrate this upon the 

 eggs a non-conducting 

 lining and hollow shape 

 is given to the nest. 

 By this means the de- 

 velopment of the egg 

 is ensured, and the 

 nestlings are protected 

 until they have de- 

 veloped a heat mechan- 

 ism sufficiently strong 

 and pliable to enable 

 them to lead a free life, 

 at least during the 

 warmth of the day. 

 A similar principle 

 underlies the nests of 

 mammals, of which 

 that of the harvest 

 mouse is perhaps the 

 best example (fig. 45). 

 That vast collection 

 of artificers which we 

 name insect, exhibits 

 in the care of its young, as in every other pursuit, 



FIG. 45. Harvest Mouse and Nest. 

 (From a specimen in the Man- 

 chester Museum.} 



