44 MANUAL OF ZOOLOGY. 



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, 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 princi- 

 pal bathymetrical zones namely, the littoral and the submarine. 



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 na- 

 ture 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 : 



1. The rocks which compose the crust of the earth have 

 been formed at successive periods, and may be roughly divided 

 into aqueous or sedimentary rocks, and igneous rocks. 



2. The igneous rocks are produced by the agency of heat, 

 are mostly unstratified (i.e., are not deposited in distinct layers 

 or strata), and, with few exceptions, are destitute of any traces 

 of past life. 



3. The sedimentary or aqueous rocks owe their origin to 

 the action of water, are stratified (i.e., consist of separate layers 

 or strata), and mostly exhibit " fossils " that is to say, the 

 remains or traces of animals or plants which were in existence 

 at the time when the rocks were deposited. 



