4 THE LIVING WORLD. 



any means have a continuous record of geological 

 ages. The layers of rocks are the positive points 

 of history, but they are separated by blank periods 

 of which no history has been preserved, and of 

 whose duration even we can get no estimate, ex- 

 cept that they were extremely long. These periods 

 must forever be blank, and it was during these 

 periods unfortunately that the greatest changes in 

 the history of life occurred. Secondly, fossils can 

 give us history of those animals alone which have 

 had a hard skeleton. The hard parts of animals may 

 readily be preserved as fossils, but not the soft parts. 

 Consequently, of the orders of animals having no 

 skeletons, fossils can tell us almost nothing. Un- 

 fortunately, too, all of the early forms of life agree 

 in having slight hard parts or none. The Protozoa, 

 Coelenterata, and Vermes have left fossil records in 

 only a few cases. We are now learning further that 

 the animals which formed the beginning of the large 

 groups were usually without skeletons. The early 

 coelenterates, mollusks, and even vertebrates proba- 

 bly had no skeletons hard enough for preservation. 

 Again, it is evident that the early representatives of 

 any type were few in number, and their chance of 

 preservation was, therefore, slight. Hence it fol- 

 lows that fossils can only in exceptional cases tell 

 us anything of the early history and development 

 of groups. Further, since the skeletons alone are 

 preserved, fossils can tell us little of the develop- 

 ment of internal structure of animals, and this is, 

 after all, probably, the principal feature of import- 

 ance^. for in most cases it seems to have been changes 



