DISTRIBUTION. 37 



The vertical or batJiymetrical distribution of animals relates 

 to the limits of depth within which each marine species of ani- 

 mals 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 com- 

 prised by that zone. Generalising on a large number of facts, 

 naturalists have been able to lay down and name certain de- 

 finite 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. 



In addition to the preceding forms of distribution, the zoolo- 

 gist 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 la- 

 teral 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 unstrattfied (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. 



4. The series of aqueous rocks is capable of being divided 

 into a number of definite groups of strata, which are technically 

 called 'formations.' 



5. Each of these definite rock-groups, or 'formations,' is 

 characterised by the occurrence of an assemblage of fossil re- 

 mains more or less peculiar and confined to itself. 



6. The majority of these fossil forms are ' extinct ; ' that is 

 to say, they do not admit of being referred to any species at 

 present existing. 



7. No fossil, however, is known, which cannot be referred 

 to one or other of the primary subdivisions of the Animal 

 Kingdom, which are represented at the present day. 



8. When a species has once died out it never reappears. 



