OF EVOLUTION. 



oldest at the bottom and ending with the newest on 

 top. I have so arranged it that each vertical inch 

 of its surface covers 2,000 feet thickness of deposit 

 belonging to each of the several periods of time, 

 the maxima of thickness occurring at any one part 

 of the earth's surface having been selected.* 

 You can thus determine for yourselves the rela- 

 tive values, as measured by the thickness of the 

 several deposits, of the different periods of time, 

 an important consideration in dealing with the life- 

 histories of animal groups. Now, when we seek to 

 investigate the life -histories of the different periods 

 indicated on this chart, we are immediately struck 

 by the very remarkable progression of the animal 

 forms distinctive of those periods. Instead of meet- 

 ing with a promiscuous association of animals of 

 lowest and highest organization, we find a gen- 

 eral advance in structural type from beginning to 

 end. It is true, we cannot in all cases indicate that 



* In the following table the relative thicknesses are 

 expressed in feet, and the measure referred to in the text 

 has necessarily been omitted. 



