72 PAST HISTORY OF ANIMALS. 



Tabular survey of the appearance of animals in time. — 

 Such tables as that on the opposite page are apt to be very 

 misleading, not only because our knowledge is very incom- 

 plete, but because it is very difficult to express accurately 

 what we do know. Thus, we cannot readily give on a page 

 any idea of the supposed relative length of the different 

 periods ; nor have we any knowledge of the very earliest ages 

 when life began. Moreover, we have not attempted, as 

 might be done by swellings and thinnings of the lines, to 

 express how the different classes seem to have waxed and 

 waned in the course of history. But the general result is 

 clear. Many types of Invertebrates have their first records 

 in Cambrian rocks, before Fishes appeared. Fishes precede 

 Amphibians, Amphibians are historically older than Reptiles, 

 and Reptiles are much older than Birds. There is no 

 shadow of doubt on the fact that in the course of ages 

 higher and higher types appeared. 



