422 LAND ANIMALS 



less than in near-by fields and 3.2° less if one considers only the 

 vegetative months. Fiefd temperatures are lower at night than those 

 in the forest, and rise higher during the course of the day. In the 

 winter, the heat of decay of the humus, together with the increased 

 insulation furnished by fallen leaves, keeps the temperature of the 

 forest floor above that of the ground in the open country ; at an air 

 temperature of —7° to —10°, the temperature under the leaves is still 

 +2° and +3 01 and so provides suitable winter quarters for many 

 forest inhabitants. The temperature differences between forest and 

 open country are still more important in the tropics, where the lower 

 strata of the forest provide a very constant temperature, while near 

 by in the open country there may be abrupt diurnal changes. 2 Since 

 the leafy covering prevents sunlight from falling directly on the soil, 

 its moisture is maintained much longer ; and since there is great evap- 

 oration from the leaves, a high moisture content is assured in the air. 

 Below the forest canopy this cannot be carried away readily by the 

 winds since the trees act as effective windbreaks. These conditions per- 

 mit stenothermal and stenohygric animals of many sorts to become 

 forest dwellers; snails, myriapods, land isopods, and amphibians every- 

 where, and in tropical forests, land planarians and terrestrial leeches ; 

 the forest-dwelling reptiles have less effective adaptations against 

 evaporation than those of open territory. 3 



The denser the forest, the less air currents penetrate into it; winds 

 are entirely absent in thick tropical forests of tall trees. 2 This is of 

 importance for many forest inhabitants. Forest butterflies may have 

 weak powers of flight without being injured by the wind. So we find 

 that the satyrids, many geometrids and tortricids in temperate Amer- 

 ica, and the tropical genera Morpho, Hestia, and many others are 

 weak fliers; the strong-flying sphingids or, in the African tropics, 

 Charaxes, are not excluded from the forest. Mammals, on the other 

 hand, are not helped by their sense of smell to the same degree in 

 forested as in open country ; in the former no breath of air carries the 

 scent of their prey or of their enemies. 



Similarly, the dense forest offers hindrances to vision. Many ani- 

 mals that do not secure their food in the forest come to it for conceal- 

 ment, and at night. The importance of the sense of sight as a means 

 of distance orientation is decreased because of the poor visibility in 

 the forest. For this reason, vultures, which as carrion feeders require 

 the freedom of view of open lands, do not occur in the forest, though 



