GEOGRAPHICAL DISTRIBUTION 323 



would not have been left as they are now marsupials and 

 parrots. 



It is only when barriers have shut out competition that 

 simple or unspecialized types abound. The larger the land 

 area and the more varied its surface, the greater is the stress 

 of competition and the more specialized are the characteristic 

 forms. As part of this specialization is in the direction of 

 hardiness and power to persist, the species from the large areas, 

 as a whole, are least easy of extermination. The rapid multi- 

 plication of rabbits and foxes in Australia, when introduced 

 by the hand of man, shows what might have taken place in this 

 country had not impassable barriers of ocean shut them out. 



Each of these great realms may be indefinitely subdivided 

 into provinces and sections, for there is no end to the possi- 

 bility of analysis. No farm has exactly the same animals or 

 plants as any other, as finally in ultimate analysis we find that 

 no two animals or plants are exactly alike. Shut off one pair 

 of animals from the others of its species, and its descendants 

 will differ from the parent stock. The difference increases 

 with time and with distance so long as the separation is main- 

 tained. Hence new species and new fauna or aggregations of 

 species are produced wherever free diffusion is checked by any 

 kind of barrier. 



In like manner, we may divide the ocean into faunal areas 

 or zones, according to the distribution of. its animals. For 

 this purpose the fishes probably furnish the best indications, 

 although results very similar are obtained when we consider 

 the mollusks or the Crustacea. 



The pelagic fishes are those which inhabit the open sea, 

 swimming near the surface, and often in great schools. Such 

 forms are usually confined to the warmer waters. They are for 

 the most part predatory fishes, strong swimmers, and many 

 of the species are found in all warm seas. Most species have 

 special homing waters, to which they repair in the spawning 

 season. To the free-swimming forms of invertebrates and 

 protozoa, found* in the open ocean, the name Plankton is ap- 

 plied. 



The bassalian fauna, or deep-sea fauna, is composed of 

 species inhabiting great depths (from 2,500 to 25,000 feet) in 

 the sea. At a short distance below the surface the change in 

 temperature from day to night is no longer felt. At a still 



