88 THE LIVING WORLD. 



the teachings of embryology as indicating the past 

 history of animals, we may regard the general facts 

 of the derivation of the Gastrasa from the unicellu- 

 lar animals, and the subsequent modification of its 

 descendants into the different sub-kingdoms which 

 soon filled the world, as substantially proved. 



We come now to the ages represented by fossi- 

 liferous rocks, and immediately the record is much 

 more definite. When we strike the fossiliferous 

 rocks we seem to be dealing with a more tangible 

 subject. The animals which were buried in the 

 ancient muds and sediments, and which we are now 

 unearthing, were actual animals and not hypotheti- 

 cal ones. With all of the cogency of argument from 

 the embryological standpoint, it cannot be denied 

 that the steps in the history of animals which it 

 points out are more or less hypothetical, and the 

 stages in the history of life of which it speaks 

 are stages of type only. But when we take a 

 fossil in our hands, we cannot question that the 

 fossil was once an actual animal, and an inhabitant 

 of the world in the earlier ages. It is now possible, 

 moreover, to determine many details in regard to 

 early life, for by means of fossils we deal with actual 

 animals, and not simply with structural types. For 

 reasons already pointed out, however, it is still 

 impossible to obtain connected history. 



The Geological Ages. 



Before proceeding with the history of animals as 

 taught us by fossils, it will be necessary to summarize 

 briefly the geological ages. The accompanying figure 



