DISTRIBUTION IN TIME. 53 



supply of food, also, and the nature of the habitat, are im- 

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

 recent facts, it would perhaps be advisable to adopt the views 

 of Dr Gwyn Jeffreys, and to consider that there are only two 

 principal bathymetrical zones namely, the littoral and the 

 subjnarine. The researches of the Challenger Expedition 

 have also shown that at depths beyond 500 fathoms, the fauna 

 presents essentially the same features all over the world, deep- 

 sea genera usually possessing a cosmopolitan range. 



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. 



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 

 remains 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. 



9. The older the formation, the greater is the divergence 

 between its fossils and the animals and plants now existing on 

 the globe. 





