96 THE LIVING WORLD. 



If we examine this fauna a little more closely, its 

 high state of diversity appears to be even more 

 startling. Not only were all of the sub-kingdoms 

 represented, but, omitting the vertebrates, all but 

 two of the classes as well. Two thirds of the 

 orders of invertebrates, and quite a large number of 

 the families which are in existence to-day, were then 

 developed. It may be said that all of the inverte- 

 brates had already developed their large trunks, and 

 the subsequent ages have in most cases only sufficed 

 to elaborate the minor branching of the trunks thus 

 formed. The Archean (i) age had been sufficient 

 for the divergence of the types of which embryo- 

 logical teachings have so plainly given evidence. 



A few figures will illustrate the fact just men- 

 tioned, that the divergence of type before the Silurian 

 (2) age produced greater modifications than those 

 that have occurred since. Of the sub-kingdoms of 

 animals, all were in existence during the Silurian. 

 Of next smaller groups, the classes, all were in 

 existence whose hard parts enabled them to be pre- 

 served, with the exception of one or two small classes, 

 whose shell is but poorly adapted for preservation. 

 Of the orders, of animals again, that have left any 

 fossil records, thirty-four are found in the Silurian 

 rocks, nineteen being in the Primordial, (i. e., at the 

 very bottom of the series), while only twenty-five 

 orders have appeared in the more recent rocks. 

 Since some of these twenty-five orders have only 

 slight skeletons, their preservation must be regarded 

 as accidental, and their absence from the Silurian (2) 

 rocks does not prove that they did not then exist. 



