78 



ANALYSIS OF THE ENVIRONMENT 



biological fitness. The other side may be 

 similarly stated: through adaptation the special 

 characteristics of the organic come to lit the 

 special characteristics of a particular environ- 

 ment, to fit, not any planet, but a little corner 

 of the earth." 



VARIATIONS IN SPACE 



The division into aquatic and terrestrial 

 organisms or habitats is primary for ecol- 

 ogy. The distribution of large bodies of 

 water is important, not alone in detennin- 

 ing the general outlines of the biogeography 

 of the world, but also in the regulation of 

 temperature and rainfall. Biogeographically, 

 the oceans provide highways for the dis- 

 persal of marine organisms; at the same 

 time they are barriers for animals of the 

 land, of fresh waters, and even for many 

 inhabitants of the shallow, inshore waters 

 of the sea. The present day distribution of 

 plants and animals depends both on the 

 existing configuration of bodies of land and 

 of water and upon the past history of these 

 configurations. 



Here we come squarely upon an active 

 controversy that centers about the possible 

 existence of oceanic land bridges. In their 

 more extreme forms, the geological prin- 

 ciples of the relative permanence of the 

 present ocean basins, based especially on 

 the principle of isotasy, are sharply op- 

 posed to theories of transoceanic land con- 

 nections or to Wegener's idea of continental 

 drift. The issues involve such matters as 

 continental and insular isolation, the loca- 

 tion and duration of routes of travel, and 

 the methods of dispersal of organisms in 

 general and in particular. 



The distribution of salts in water is 

 fundamental for large-scale distinctions in 

 the distribution of aquatic organisms. The 

 highly saline lakes or lagoons, the oceans, 

 and the fresh waters of the world form a 

 series of distinct environments. Gradual 

 transitions occur, and brackish water makes 

 a well-known transition between marine 

 and fresh-water environments. 



The general principles and facts concern- 

 ing the broad temperature zones of the 

 world are well known. It is not so generally 

 appreciated that the present zonal climate 

 is a recurrent, relatively transitory phase 

 of climatic history. Throughout much of 

 the time that the earth has been inhabited, 

 the continents have stood lower in relation 

 to sea level than they are at present, and 



relatively mild temperatures have exLended 

 into subpolar regions. In other words, the 

 strong zonal provincialism of present day 

 temperature belts has usually been re- 

 placed by a broad uniformity. One of the 

 unsolved problems of modern world climate 

 is whether we are now in another inter- 

 glacial period or are moving toward the 

 general amelioration of world climates. 



The phases of temperature zonation 

 concerned with life zones will be considered 

 in more detail later (p. 114). Meantime, 

 it should be lecalled that many regional 

 or local factors act to modify the tempera- 

 ture in a given region from that to be ex- 

 pected on an idealized globe. Distance from 

 the ocean is one of the modifying factors. 

 The ocean is the great temperature regu- 

 lator of the world. Islands and coastal 

 areas, in general, undergo relatively slight 

 temperature fluctuations as contrasted with 

 the extremes found in the midcontinental 

 climates at the same latitude. This effect is 

 quite apart from a second important tem- 

 perature modification brought about by 

 ocean currents. The ameliorating action of 

 the Gulf Stream upon the temperature of 

 northern Europe contrasts with the chilling 

 produced by the Labrador Current at sim- 

 ilar latitudes along the northeastern coast 

 of America. Winds exert important effects 

 on the temperature and rainfall of a given 

 region. Thus, the prevailing westerly winds 

 accentuate the ameliorating eflFect of the 

 Gulf Stream on the chmate of northwestern 

 Europe. 



Tropical and subtropical temperatures 

 are much more restricted along the western 

 coasts of the continental land masses than 

 they are on the eastern side. This restric- 

 tion is brought about either by an upwell- 

 ing of deeper, cold ocean water or by polar 

 currents, or by both acting together. Trop- 

 ical littoral animals are found, for example, 

 only from the northern coast of Peru, 5 de- 

 grees or less south of the equator, north- 

 ward to northern Mexico or southern Cal- 

 ifornia, a total distance of about 33 to 39 

 geographic degree (Ekman, 1935) (see 

 Fig. 3). On the eastern side of the Amer- 

 icas, the comparable littoral formation ex- 

 tends from Cape Hatteras and the Bermu- 

 das at 35 degrees north latitude to Rio de 

 Janeiro or even to the mouth of the Plata 

 river at 35 degrees south latitude. The situa- 

 tion is similar on the two coasts of the 

 African-Eurasian land mass and on those of 



