460 



ESSENTIALS OF ZOOLOGY 



solids and oxygen. Air is habitable only temporarily and animals 

 spending time there must return frequently to water or soil. Free 

 interaction between the three components of the earth is made pos- 

 sible by the fact that the lithosphere is not of exactly uniform 

 diameter so that certain portions project above the hydrosphere, 

 forming continents (and islands). These elevated areas constitute 

 a rather small proportion of the whole surface of the earth, but it 

 is with these areas that we shall be concerned, not because there 



Fig. 198. — Distribution of "precipitation effectivity" on an ideal continent. (Modified 

 after Thornthwaite. Drawn by Edward O'Malley.) 



is no life in the submerged areas, but because the space available 

 in this chapter is too small for us to consider the great subject of 

 marine ecology. 



The inclination of the earth's axis of rotation to the plane of the 

 earth's orbit about the sun and the relative positions of the conti- 

 nents are factors which influence the distribution of climates. The 

 two principal climatic factors are, of course, temperature and mois- 

 ture. If the effect of altitude is not considered, the former is distrib- 

 uted rather uniformly, so that the familiar torrid, temperate, and 



