428 LAND ANIMALS 



enemies ; it is also a refuge from climatic extremes. The relatively con- 

 stant humidity and the reduced temperature variations permit a sur- 

 vival in forests of northern Europe and of the United States of species 

 that occupied open country in glacial times; thus the modern German 

 snail fauna in the forests resembles that of non-forested Lapland and 

 north Russia. 25 



Forest types. — The botanist recognizes a variety of forms of 

 forest which are of importance to the student of animal geography only 

 as they are associated with distinct animal communities. Animals 

 clearly react to the following: 



I. Forests characterized by thin leaves. 



1. Evergreen, the tropical rain-forests. 



2. Deciduous. 



a. With alternating dry and wet seasons. 



b. With alternating warm and cold seasons. 



II. Forests characterized by broad thick leaves; the sclerophyl- 

 lous forests of regions of winter rain. 

 III. Forests characterized by narrow thick leaves; the coniferous 

 forests. 



The tropical rain-forests. — A vast forest girdle encircles the earth 

 between the tropics, extending across northern South America, central 

 Africa, parts of India and the Malay peninsula, the East Indies, and 

 northern Australia. This rain-forest reaches its largest continuous ex- 

 tent in South America, where half of the continent is covered by a 

 tropical rain-forest, approximately 4000 by 3000 km. in extent. These 

 rain-forests vary greatly in different regions, being affected by the 

 height above sea level, the character of the soil, the regularity and 

 the amount of rainfall, and the amount of insolation, all of which affect 

 the species of trees to be encountered in any particular fraction of this 

 world forest. Despite these differences, there is an astonishing similarity 

 throughout this tropical region with regard to the general growth- 

 form, the luxuriance of the vegetation, and the great multiplicity of 

 species among the trees present. Conditioned by these factors one finds 

 here reduced illumination, high and relatively constant humidity and 

 temperature, a lack of air currents, and, along the margins or around 

 openings, a maximum of impenetrability. As might be expected from 

 these conditions, the animal life of the tropical rain-forest is dis- 

 tinguished from that of other world communities not only by its species 

 composition but also by its internal relations. 26 



The tropical rain-forest presents a vertical series of strata availa- 

 ble for animal occupancy which suggested Humboldt's description of 

 forests above forests. The lower strata may be poorly developed in 



