296 EVOLUTION AND ANIMAL LIFE 



and when the many deposits of fossils in all parts of the different 

 continents are examined and compared, it is possible to state 

 numerous general truths in regard to past life and the suc- 

 cession of animals in time. The science of extinct life is known 

 as paleontology. 



The study of paleontology has revealed much of the history 

 of the earth and its inhabitants from the first rise of the land 

 from the sea till the present era. This whole stretch of time 

 how long no one can guess is divided into eras or ages ; these 

 ages usually into lesser divisions called periods, and the periods 

 into shorter lengths of time called epochs. Each epoch is 

 more or less sharply distinguished from every other by the 

 different species of animals and plants which lived while its 

 rocks were being deposited. In the earth's crust, where it has 

 not been distorted by foldings and breaks, the oldest stratified 

 rocks lie at the bottom of the series, and the newest at the top. 

 The fossils found in the lowest or oldest rocks represent, there- 

 fore, the oldest or earliest animals, those in the upper or newest 

 rocks the newest or latest animals. 



An examination of a whole series of strata and -their fossils 

 shows that what we call the most specialized or most highly 

 organized animals did not exist in the earliest epochs of the 

 earth's history, but that the animals of these epochs were all 

 of the simpler or lower kinds. For example, in the earlier 

 stratified rocks there are no fossil remains of the backboned 

 or vertebrate animals. When the vertebrates do appear, 

 through several geological epochs they are fishes only, members 

 of the lowest group of backboned animals. More than this, 

 they represent generalized types of fishes which lack many of 

 the special adaptations to marine life that modern fishes show. 

 For this reason they bear a greater resemblance to the earlier 

 reptiles than do the fishes of to-day; in other words, they 

 were a generalized type, showing the beginnings of characters 

 of their own and other types. It is always through general- 

 ized types that great classes of animals approach each other. 



In a later epoch the batrachians or amphibians appeared; 

 in a still later period, the reptiles; and last of all, the birds and 

 the mammals, the last being the highest of the backboned 

 animals. The following table gives the names and succession 

 of the various geological periods, and indicates briefly some 

 of the kinds of animals living in each. In each of these di- 



