DISTRIBUTION. 39 



lished ; these become permanent, and " races " are produced ; 

 finally, in the lapse of time, the differences become sufficiently 

 great to constitute distinct species. 



14. DISTRIBUTION. 



Under this head come all the facts which are concerned with 

 the external or objective relations of animals ; that is to say, 

 their relations to the external conditions in which they are 

 placed. 



The geographical distribution of animals is concerned with 

 the determination of the areas within which every species of 

 animal is at the present day confined. Some species are found 

 almost everywhere, when they are said to be " cosmopolitan;" 

 but as a rule each species is confined to a limited and definite 

 area. 



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 four 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, 50 to 100 fathoms or more. 



5. To these must now be certainly added a fifth zone, 

 extending from 100 fathoms to a depth of 2500 fathoms or 

 more. 



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. 



In addition to the preceding forms of distribution, the 

 zoologist has to investigate the condition and nature of animal 

 life during past epochs in the history of the world. 



The laws of distribution in time, however, are, from the 

 nature of the case, less perfectly known than are the laws of 

 lateral or vertical distribution, since these latter concern beings 



