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 Palzeozoic 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- 
