126 Water 



factors as soil type and temperature and of the interdependence of 

 these factors with moisture it is clear that no simple relation between 

 the type of vegetation and any single physical influence can be ex- 

 pected. However, the broad correspondence between the zones of 

 vegetation and the availability of moisture will suffice to emphasize 

 the major role of the water factor in the region under discussion in 

 controlling both the soil and the plant life that develops with it. 

 The application of the P-E ratios to these distributional problems has 

 again brought into relief the fact that the balance between supply 

 and loss is the most crucial aspect of the moisture factor for the vege- 

 tation. 



The water factor on land also seriously affects the growth and dis- 

 tribution of animals either directly or indirectly. Since the range of 

 land animals is profoundly influenced by the vegetation, moisture 

 often exerts its greatest effect on animals indirectly through its con- 

 trol of the plants. For the higher vertebrates temperature is generally 

 a more important direct environmental influence than moisture. 

 However, it has long been known that races of birds and mammals 

 in warm humid regions tend to be darker in color than races inhabit- 

 ing the cooler and drier parts of the geographical range of the species. 

 This generality is known as Glogers rule, but many exceptions exist 

 and the relative effects of heat and moisture are not known. There 

 is evidence that humidity acts more through the color of the soil as a 

 background for the animals than in a direct way. The implications of 

 Gloger's rule will be discussed in more detail below in relation to 

 temperature and light. 



For amphibians and for insects and other terrestrial invertebrates 

 moisture is often of great direct importance. Many insects exhibit 

 critical dependence upon humidity conditions with sharp limits of 

 tolerance. For example, the "silverfish" {Lepisiiia saccharina), a 

 common household insect pest, finds optimum conditions for repro- 

 duction at relative humidities of 85 to 90 per cent. The newly 

 hatched nymphs die if they are subjected to relative humidities of less 

 than 70 per cent, and they also succumb if their surfaces once become 

 wet. The relationship of insects to moisture is frequently very com- 

 plex and varies greatly in different species and sometimes even in 

 different parts of the life cycle of the same species (cf. Chapman, 

 1931, Ch. 4). In the development of the flour beetle Tribolium the 

 larval stage is accelerated by an increase in relative humidity, but the 

 duration of the egg and pupa stages is unaffected by wide changes 

 in this factor (Fig. 4.16). Moisture produces entirely different ef- 

 fects upon the survival of this insect. In the larval stage increasing 



