170 THE MEANING OF EVOLUTION 



mals of to-day. The reader must not infer this to 

 mean that the animals of those days were like our 

 present animals. They were not. No one traveling 

 in a far country could find there animals as strange to 

 him as would be those of the earlier stratified rocks. 

 In these there were no fishes as we know them to- 

 day, not a single member of the frog and salamander 

 class, not a reptile, not a bird, not a mammal, and 

 probably no air-living insects. It is highly doubtful 

 whether there was any animal living upon the land 

 and breathing the air twenty-five million years ago. 



We start our study, then, at the period known as 

 the Palaeozoic era, the era of the ancient life of the 

 globe, beginning twenty-five million and ending ten 

 million years ago. The first of the three sections into 

 which this period of life is divided is known as the 

 Silurian age, the age of invertebrates. The word in- 

 vertebrate is an unscientific but convenient term under 

 which we embrace all the animals below those having 

 backbones. This period is called the age of inverte- 

 brates because, although there is an enormous wealth 

 of animal and plant life in the Silurian, there are no 

 backboned animals except the lowest kinds of fishes. 

 It was supposed for a long time that even fishes were 

 absent. Now we know they existed, but they were 

 small and inconspicuous. In this period corals were 

 wonderfully abundant, particularly in the great inter- 



