52 MANUAL OF ZOOLOGY. 



1. North America, north of Mexico. 



2. Africa, south of the Sahara. 



3. Hindostan. 



4. The remainder of the Old World (Europe, Africa 



north of the Sahara, Asia generally, but without 



Hindostan, &c.) 



Mr Sclater, basing his arrangement primarily on the distri- 

 bution of birds, divides the earth's surface into the following 

 six provinces : 



1. The Pal&arctic Province, including Europe, Africa north 

 of the Atlas Mountains, and Northern Asia. 



2. The AZtkopian Province, including Africa south of the 

 'Atlas Mountains, and Southern Arabia. 



3. The Indian Province, including Asia south of the Hima- 

 laya Mountains, Southern China, and the Indian Archipelago. 



4. The Australian Province, including Australia, Tasmania, 

 New Guinea, New Zealand, and a large proportion of the 

 islands of the Pacific Ocean. 



5. The Nearctic Province, including North America down to 

 the centre of Mexico. 



6. The Neotropical Province, including the whole of South 

 America, Central America, and Southern Mexico. 



The vertical or bathymetrical distribution of animals relates 

 to the limits of depth within which each marine species of 

 animals is confined. As a rule it is found that each species 

 has its own definite bathymetrical zone, and that its existence 

 is difficult or impossible at depths greater or less than those 

 comprised by that zone. Generalising on a large number of 

 facts, naturalists have been able to lay down and name certain 

 definite zones, each of which has its own special fauna. 



The five following zones are those generally accepted : 



1. The Littoral zone, or the tract between tide-marks. 



2. The Laminarian zone, from low water to 15 fathoms. 



3. The Coralline zone, from 15 to 50 fathoms. 



4. The deep-sea Coral zone, from 50 to 100 fathoms. 



5. To these must now be certainly added a fifth or "Abys- 

 sal " zone, extending from 100 fathoms to a depth of 3000 or 

 4000 fathoms. 



Recent researches, however, have rendered it certain that 

 after a certain depth, say 100 fathoms, the bathymetrical dis- 

 tribution of animals is conditioned not by the depth, but by 

 the temperature of the water at the bottom of the sea. Similar 

 forms, namely, are always found inhabiting areas in which the 

 bottom-temperature is the same, wholly irrespective of the 

 depth of water in the particular locality in question. The 



