80 



ANALYSIS OF THE ENVIRONMENT 



northern semidesert which, in California 

 and around the Mediterranean, is an area 

 of winter rain and summer drought. 



5. To the north lies a region of moderate 

 rainfall that supports either deciduous 

 forests or grasslands in its southern phases 

 and a round-the-world belt of coniferous 

 forests at the north. 



6. Farther north there is the tundra, 

 where the rainfall is characteristically scanty 

 and where even the small amount that does 

 fall is physiologically unavailable during the 

 greater part of the year. 



7. Finally, as far as land is concerned, 

 there are the well-developed polar ice caps 

 in Greenland and Antarctica. 



A similar set of conditions can be recog- 

 nized in the southern hemisphere, although, 

 associated with the smaller size of the con- 

 tinental land masses, the rainfall zonation 

 is not so diagrammatically developed ex- 

 cept for the polar ice cap in Antarctica. 



The distribution of rainfall is strongly 

 a£Fected by mountain ranges. When these 

 extend east and west, as do the Himalayas, 

 the combined rainfall and temperature zon- 

 ation is accentuated. When the mountains 

 extend north and south, as do the Rocky 

 and the Andes Mountains, a secondary pat- 

 tern of rainfall distribution is established 

 which, as will be discussed in more detail 

 later (p. 145), runs at right angles to the 

 global temperature zones. 



The geography of temperature and rain- 

 fall and of associated factors exerts a strong 

 influence upon the distribution of species 

 of plants and animals and of biotic com- 

 munities that is strikingly shown on the 

 land. Temperature also exerts a strong pri- 

 mary influence on the distribution of 

 marine organisms. The effect of rainfall on 

 marine life is mainly indirect and acts 

 through modification of salinity. Areas of 

 dilution occur along shores and particu- 

 larly near the mouths of the large tropical 

 and subtropical rivers where the great in- 

 flux of fresh water, together with the silt it 

 carries, inhibits the growth of coral reefs. 

 The opposite effect may be noted near des- 

 ert areas, most strikingly in the Red Sea, 

 which shows the effect of its location in the 

 great northern desert belt by the high salin- 

 ity of its waters, 46.5 per mille, as con- 

 trasted with the 35 per mille characteristic 

 of the surface waters of the open ocean. 



The surface sahnity in the three major 

 oceans, and for these combined, varies from 



a standard value in direct proportion to 

 the difference between evaporation and pre- 

 cipitation in the area under consideration. 

 Although modified by mixing with water 

 from 400 to 600 meters down, the differ- 

 ence between evaporation and precipitation 

 is of primary importance (Sverdrup, John- 

 son, and Fleming, 1942, p. 124). 



Especially on land, other environmental 

 factors are also differentially distributed 

 and are important in ecological geography 

 and physiology. They are usually subsidiary 

 to the temperature-rainfall complex. Some 

 of the more important ones include the 

 length of day and the environmental con- 

 ditions associated with altitude and sub- 

 strate. 



The distribution of soil types forms an 

 important basis of endemism in continental 

 areas, while the presence or absence of 

 traces of copper, cobalt, or selenium, tc 

 name no more, in the soil may have impor- 

 tant ecological effects (p. 221) (Godden, 

 1939). 



VARIATIONS IN TIME 



Some major variations in time have been 

 outhned in the preceding pages, especially 

 those changes that have accompanied the 

 evolving fitness of the physical world to 

 support life. The present discussion will 

 center about (a) changes in chmate on the 

 earth during geological time and (b) more 

 recent and present day periodicities. 



Geological Climates 



Physical and biological evidence both 

 indicate that climate during historical times 

 is a poor key to the more usual world cli- 

 mates of the past. Probably less than 1 per 

 cent of geological time has approximated 

 the essentially glacial climatic pattern that 

 is familiar to us. Other aspects of the late 

 Cenozoic and Recent epochs are abnormal. 

 Mountains are more numerous and stand 

 higher; continents are larger; there are more 

 volcanoes; and earthquakes come more fre- 

 quently than they did during the great 

 stretches of geologic time. We are living in 

 a period of geological revolution, of crustal 

 unrest, such as occurred on a full scale 

 between the Proterozoic and the Paleozoic 

 eras and was repeated between the Pale- 

 ozoic and Mesozoic eras (Brooks, 1926; 

 Russell, 1941). 



Generally speaking, crustal stability, 



