BIOGEOGBAPHY 



between the two regions, and one might imagine that it may have been 

 complete at one time. But the channels between some of the islands are 

 very deep, a fact which fa\'ors permanence, or at least very long duration. 

 Nonetheless, the physical conditions within one region may be so closely 

 similar to those of another as to be indistinguishable, so long as biotic fac- 

 tors are neglected. Thus the climate and physiography are much alike 

 over large areas of South America and Africa. It can scarcely be doubted 

 that each presents habitats quite suitable for the plants and animals of 

 the other. Yet they have few organisms in common, and those they do 

 have may often be explained as isolated survivors from once world-wide 

 groups. Thus the Dipnoi (lungfishes) are now represented by only three 

 living genera, Neoceratodiis in Australia, Protopterus in Africa, and Lep- 

 idosiren in South America. The African and South American forms belong 

 to the same family, while the Australian form is the sole member of its 

 family. If only the living species be considered, it appears that the fishes 

 of the southern continents have an especial relationship, despite the great 

 ocean barriers which separate them. However, the fossil record shows that 

 the lungfishes were once of world-wide distribution. They have long since 

 become extinct in the face of competition with better adapted forms in 

 most parts of the world, but the southern continents have been a last 

 refuge of survival for these and so many other primitive forms. 



That the flora and fauna within any one region show a certain con- 

 sistency is to be expected on the basis of any theory of origin. Increasing 

 populations, with their subsequent dispersal throughout the available ter- 

 ritory, should achieve this. But the fact that many plants and animals are 

 excluded from lands for which they are eminently well suited is difficult 

 to explain on any theory other than the evolutionary one. This is exempli- 

 fied by the problem of widely separated populations of the same or closely 

 similar species, which has already been discussed above. It is difficult to 

 understand why, if these represent independent creations of the same 

 species, they should not be placed in similar parts of different regions. 

 Why, for example, should both populations of magnolias be located in the 

 Holarctic Region, when both the Oriental and the Neotropical Regions 

 present eminently satisfactory habitats? And why should generally similar 

 organisms be grouped together in such regions when suitable habitats for 

 almost any organism can be found outside its own region? 



ECOLOGICAL ZONES IN THE OCEAN 



Distinct biotic regions are not limited to the land masses of the world. 

 Although there is a degree of physical continuity between the oceans of 

 the world, the habitats presented in different parts are quite different, and 

 so ecological factors produce barriers within the oceans (Figure 4). On 

 every shore, there is a narrow strand which is alternately covered and 

 exposed by the tides. Beyond this intertidal or littoral zone, there is the 

 broad, gently sloping continental shelf, the higher portions of which form 

 the continental islands. The seas over the continental shelves are generally 

 shallow, not over 100 fathoms (600 feet) deep, and they comprise the 



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