INTEODUCTION. XXIX 



instances, exaggerated. Other natural barriers, 

 even more powerful, interpose to prevent the too 

 wide-spread diffusion of both plants and animals. 

 The several facts which may be deduced from 

 observation of the distribution of animals in Space, 

 fall under the three following categories : — 



1. The relations of animals to the elements in 

 which they live. 



2. Their lateral (geographical) distribution. 



3. Their vertical (bathymetrical) distribution. 

 The distribution of animals in Time forms a 



distinct subject of inquiry, the consideration of 

 which can only be entered on with profit by those 

 who possess some acquaintance with the science 

 of physical geology. It will, therefore, be suf- 

 ficient here to state the following generalisa- 

 tions : — 



Every species has a definite distribution in 

 Time. 



Each of the great geological epochs contained 

 an assemblage of forms, different from one 

 another, and the present. 



In the older geological formations, there ex- 

 isted representatives of each of the five great 

 divisions of the animal kingdom. 



The animal remains found in these ancient 

 formations, though referrible to the same classes, 

 belonged, in many cases, to orders, families, 

 genera, and species, different from those now 



