HIGH SCHOOL ZOOLOGY. 281 



mit, but the most effectual barriers are high niountain-cliains 

 and deep seas. By mapping out the ranges of different species 

 it is jiossible to mark out certain zoogeographical regions in 

 which the simultaneous occuri-ence of the same or nearly allied 

 species confers a certain similarity of appearance or facies to the 

 fauna. A glance at these regions, as laid down by Dr. Wallace 

 in the accompanying map, will shew that they agree pretty 

 well with our ordinary geographical divisions, except that great 

 natural barriers cut off the Ethiopian region from North Africa, 

 the Oiiental from the Palsearctic, and the Australian from the 

 Oriental ; these are the Sahara, the Himalaya, and the very 

 deep sea along " Wallace's line," between Bali and Lombok, 

 Boineo and Celebes. This is certainly a most instructive 

 illustration of the effect of a deep-sea barriei', and of the fact 

 that mere climate does not determine geographical distribution. 

 From Bali to Lombok is hardly twenty miles, and yet there is 

 more difference as to the fauna between these two islands, the 

 climate of which is practically identical, tiian there is between 

 Canada and Florida. 



17, Although the distribution of aquatic animals admits also of 

 geographical treatment, it is obvious that the barriers to their 

 spi-eading far and wide must be of very different chai-acter, and, 

 apart from those forms confined to the shore by limited loco- 

 motive powers, must be largely due to ocean currents, tempera- 

 ture and other surrounding conditions. Other elements of 

 interest, however, exist in connection with them— their occur- 

 rence in fresh or in salt water, and their bathymetrical distri- 

 bution — i.e., the depth at which they occur. 



The greatest profusion of life is found in the shallow waters 

 of the shore- zone, but recent observations have shown that it 

 also abounds in the depths of the sna, even down to four or five 

 miles. Again, there are so-called pelagic forms which live on 

 the surface-waters of the open sea, and are only occasionally 

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