DISTRIBUTION. 4I 



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

 area. Not only are species limited in their distribution, but it 

 is possible to divide the globe into a certain number of geo- 

 graphical regions or " zoological provinces," each of which is 

 characterised by the occurrence in it of certain associated forms 

 of animal life. It is to be remembered, however, that the 

 zoological provinces of the present dsLy by no means corre- 

 spond with those of former periods, and that they have only 

 existed as such since comparatively recent times. 



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, naturaUsts 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, ex- 

 tending 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. The 

 supply of food, also, and the nature of the habitat, are important 

 elements of the case. In the light, therefore, of these recent 

 facts, it would perhaps be advisable to adopt the views of Mr 

 Gwyn Jeffreys, and to consider that there are only two prin- 

 cipal bathymetrical zones — ^namely, the littoral and the sub- 

 marine. 



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 

 which we are able to examine directly. The following are the 

 chief facts which it is necessary for the student to bear in mind': 



