120 THE LIVING WORLD. 



between the ages, we are of course prepared to find 

 that each age is characterized at its outset by a new 

 set of species, and in some cases by a distinctly 

 new fauna. Remembering, then, that all breaks in 

 the history occur where there are breaks in the 

 record, and that there are no breaks in the history 

 where the record is complete, we may unhesitatingly 

 conclude that the real history of life has been one 

 of continual slow change, without breaks, with each 

 age passing imperceptibly into the next. 



Animal Types the Same To-Day as in Earlier Times. 



With all of this extinction of old forms, it is 

 remarkable that since the Silurian (2) no great 

 type of animals has disappeared. All of the sub- 

 kingdoms in existence in the Silurian are still in 

 existence to-day, and the same is equally true of the 

 classes and nearly so of the orders. It is marvel- 

 lously interesting and surprising to find that all of 

 the fossils found in the rocks deposited so many 

 millions of years ago can be readily placed with one 

 or another of the divisions of animals that are in 

 existence to-day. The fact gives us a forcible lesson 

 that the animal kingdom is one, and that during all 

 the history of the world it has been a unit. It tells 

 us that the same laws in force to-day were in force 

 millions of years ago in the world. It indicates that 

 some bond has united the animals of the rocks with 

 those of our seas and lands ; and this unity gives us 

 one of the strongest arguments for the belief that 

 heredity and evolution have been the laws presiding 

 over the development of the animal kingdom. 



