CHAPTER XI 



THE DISTRIBUTION OF ANIMALS 



In the preceding chapter we have considered the distribution 

 of animals in time ; we may next glance at their distribution on 

 the earth to-day, taking first their vertical or bathymetrical dis- 

 tribution, and then their horizontal or geographical distribution. 

 The animals which collectively inhabit any region constitute its 

 fauna, just as the plants in any particular place form the flora 

 of that place. Concerning the vertical distribution of animals, 

 or as we may w r ith equal propriety call it, their habitat, we have 

 to do with marine animals, fresh-water animals, terrestrial ani- 

 mals, and aerial animals. 



A few facts concerning the physical geography of the ocean 

 are necessary to even a brief consideration of its animal life. 

 The ocean is far from being of uniform depth. The deepest 

 points known are much restricted in area and measure about 

 9000 meters ; the greater portion of the ocean bottom is from 

 5000 to 6000 meters below the surface. The portion of the 

 continents above sea level does not pass abruptly to this great 

 depth as a rule, nor is there a gradual slope from the shore line 

 to the deep sea. Instead, the land passes under the water in a 

 gentle slope to a depth of about 200 meters ; then it plunges 

 very precipitously as a rule to a depth of 2000 meters. The 

 portion of the land covered thus by relatively shallow water is 

 called the continental shelf, and varies much in width at various 

 points on the different continents. Thus the shelf on the east- 

 ern coast of North America is relatively broad, on the western 

 coast very narrow. 



Not only is there great variation in the general topography 

 of the ocean bed, but the nature of the deposits which cover it 

 is very different in different regions. The portion of the ocean 

 bottom which adjoins the shore is covered for a variable distance 



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