GENERAL LAWS OF DISTRIBUTION. 189 



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 oper- 

 ation 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 

 ^arth. 



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 Moorish 

 coast, the latter being merely an appendage to the fauna 

 of Europe. But the sea effects the most complete limita- 

 tion. The depths of the ocean are quite as impassable 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 

 from one fauna to another is made insensibly. Thus, in 

 passing from the arctic to the temperate regions of North 



