DIVERSITY OF SILURIAN LIFE. 1 03 



The Diversity of Silurian Life. 



Still another difficulty has arisen in connection 

 with Silurian fauna, closely related to the one just 

 mentioned. Our paleontologists have within the 

 last thirty years had most wonderful success in 

 tracing existing types back to the oldest rocks; a 

 result which the theory of descent, with modifica- 

 tion, would not lead us to expect. It must be ad- 

 mitted that upon this theory we should expect to 

 find the Silurian fauna radically different from that 

 existing now. Taking into account the enormous 

 time that has elapsed since then, it would seem that 

 the whole organic world would have become altered. 

 The amount of modification possible in this time is 

 shown by the fact that the entire group of verte- 

 brates has developed into its present condition 

 since the Silurian. It will probably be admitted 

 by every one that, did we not have any actual knowl- 

 edge of the Silurian, a hypothetical fauna would 

 be assumed differing entirely from that of to-day. 

 It might be admitted that there were even then 

 specialized groups ; but it would be supposed that 

 they had long since passed away. Perhaps a few 

 lingering forms might be thought to have survived, 

 but in general the old species, genera, orders, and 

 classes would have become replaced by others. In 

 short, just such a fauna would be assumed as is 

 now thought to belong to the pre-Silurian times. 



It is true that the fauna of to-day is different 

 from that of the Silurian age, but the difference lies 

 chiefly in the species and genera. We find that in the 

 Silurian fauna are represented all of the sub-king- 



