DISTRIBUTION. 413 



GEOLOGICAL DISTRIBUTION. 



Geographical distribution treats of the distribution of 

 animals in space; there is also a distribution in time. 

 The animals of past ages have left their record in the rocks, 

 and a study of these fossils shows that in the past the ani- 

 mals in any region were more or less different from those 

 in the same region to-day. Thus at the time of its dis- 

 covery America contained no elephants, camels, horses, 

 or rhinoceroses, yet the rocks of our western states have 

 revealed the skeletons of all of these animals. 



Geologists have classified the rocks according to their 

 age, and by studying the fossils which they contain we 

 get not only an idea of the changes in a region, but also 

 of the progress of life on the earth as a whole. Going 

 from the older to the newer rocks the following are the 

 main subdivisions of time recognized by geologists and 

 the characteristic animals contained in each. 



I. Azoic OR ARCH^AN AGE. 

 No fossils are certainly known from this age. 



II. PALAEOZOIC AGE. 



Divided into (1) Cambrian, (2) Silurian, (3) Devonian, 

 (4) Carboniferous, and (5) Permian periods. In the 

 Cambrian rocks only invertebrates occur: Brachiopoda, 

 trilobites, molluscs, and echinoderms prevail. Corals and 

 fishes appear in the Silurian, amphibia in the Carbonifer- 

 ous, reptiles in the Permian. 



