IN TR OD UC TION. 5 



in the anatomy of soft parts which have inaugurated 

 all of the larger departures in animal history. Again, 

 the rocks in which fossils have been deposited have 

 been frequently subjected to the modifying effects 

 of heat and pressure (metamorphosis). Sometimes 

 this metamorphosis has completely obliterated what- 

 ever fossils may have been in them originally, and 

 very commonly it has so obscured their structure as 

 to render it impossible to determine very definitely 

 what they originally were. It is readily seen that, 

 the older the rocks the more liable they will be to 

 such metamorphoses, and hence the farther back 

 into history we go, the less definite becomes the 

 record. Of the most recent epochs, quite a complete 

 history is obtainable, but as we go back the record 

 becomes less and less sure, and finally it stops alto- 

 gether. Of the earliest history of life fossils give us 

 absolutely no trace, and it is even true that fossils 

 give us no satisfactory record of the early history of 

 any of the groups of animals. 



From all this it will appear that although fossil 

 history is very definite where it is obtainable, it will 

 at best be a disjointed history abounding in details 

 at some points and lacking at others. Some small 

 steps in the history will be given in the most minute 

 particulars, because of the abundance of animals 

 with skeletons to represent them, while other im- 

 mense epochs will be entirely unrecorded from the 

 lack of proper conditions to produce and preserve 

 fossils representing the period. Moreover, from the 

 great metamorphosis of the rocks of the older periods, 

 the study of fossils will give us absolutely no record 



