^^ ZOOGENESIS 1^"^ 



of the time but appear at intervals on warm and 

 sunny days. 



When the weather gets extremely cold the air, with 

 any rise in temperature, becomes extremely dry and, 

 speaking generally, dryness is more dangerous to life 

 than cold. This raises an interesting question. Why 

 do the small birds that visit us each winter from the 

 north, such as the horned larks, snowflakes, kinglets, 

 creepers and others, leave their summer homes? Is 

 it not probable that they are induced to visit warmer 

 regions not so much on account of the cold itself as 

 because of the dryness which accompanies the cold? 



For most of these birds there is quite as much food 

 available in winter in their northern homes as there is 

 with us, but the water content of that food is consider- 

 ably less. It is to be remarked that when they visit 

 us these little birds keep mainly in damp localities, 

 in low damp woodlands, about ponds and streams, or 

 near the sea coast. 



Much as the animals and plants with us pass 

 through the winter do tropical animals and plants 

 pass the dry hot season. For instance, some of the 

 lemurs in Madagascar spend the dry season in a state 

 of torpor coiled up in a cavity of a tree or in a nest 

 just as some of our squirrels spend the winter. 



In some places in the tropics at the end of the wet 

 season the trees for the most part shed their leaves, 

 and the insects almost completely disappear. A 

 photograph of such a region at this time much re- 

 sembles one of a snowless day in winter in the north. 

 Conditions are the opposite in that in one case nature 



[49^ 



