GENERAL LAWS OF DISTRIBUTION. 157 



402. This diversity upon different continents cannot de- 

 pend simply on any influence of the climate of the tropics ; 

 if it were so, uniformity ought to be restored in proportion 

 as we recede from the tropics towards the antarctic tem- 

 perate regions. But, instead of this, the differences con- 

 tinue to increase ; so much so, that no faunas are more in 

 contrast than those of Cape Horn, the Cape of Good Hope, 

 and New Holland. Hence other influences must be in ope- 

 ration besides those of climate; influences of a higher 

 order, which are involved in a general plan, and intimately 

 associated with the development of life on the surface of 

 the earth. 



403. Faunas are more or less distinctly limited, according 

 to the natural features of the earth's surface. Sometimes 

 two faunas are separated by an extensive chain of moun- 

 tains, like the Rocky Mountains. Again, a desert may in- 

 tervene, like the desert of Sahara, which separates the 

 fauna of Central Africa from that of the Atlas and the Moor- 

 ish coast, the latter of which is merely an appendage to the 

 fauna of Europe. But the sea effects the most complete 

 separation. The depths of the ocean are quite as impassa- 

 ble for marine species as high mountains are for terrestrial 

 animals. It would be quite as difficult for a fish or a mollusk 

 to cross from the coast of Europe to the coast of America, 

 as it would be for a reindeer to pass from the arctic to the 

 antarctic regions, across the torrid zone. Experiments of 

 dredging in very deep water have also taught us that the 

 abyss of the ocean is nearly a desert. Not only are no 

 materials found there for sustenance, but it is doubtful if ani- 

 mals could sustain the pressure of so great a column of 

 water, although many of them are provided with a system 

 of pores (260), which enables them to sustain a much greater 

 pressure than terrestrial animals. 



404. When there is no great natural limit, the transition 



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