Animals Before Man 



tinent again sunk beneath the sea and again 

 risen, but from the rocks and fossils we may 

 learn the story of these changes, set the former 

 boundaries of the land, and people the earth 

 with its long-vanished life. 



If the rocks from two widely separated 

 localities are found to contain the same or even 

 similar species of fossil land animals, it is to be 

 inferred that these rocks were formed at about 

 the same period of time, and that there was a 

 land connection between the two places. These 

 are very general propositions, but in actual use 

 there are several factors to be taken into con- 

 sideration, and with invertebrates the case is 

 yet more complicated. 



If the fossils are very different in their na- 

 ture, we may be sure that the rocks were sepa- 

 rated either by time or space ; and if the fossils 

 are those of mammals, they will probably tell 

 which of these two possibilities is a probability. 

 For here it may be said that the different kinds 

 of animals keep as it were different kinds of 

 time, the low animals of simple structure seem- 

 ing to change much more slowly than those 



22 



