332 STRUCTURE OF THE VERTEBRATES 



fully studied, any new region discovered could be quickly and 

 accurately identified by the remains of living organisms found 

 embedded. 



In the very nature of things the record is, and probably al- 

 ways will be, incomplete. Each year new data are added as new 

 fossils are found and studied; but at best the record can be 

 compared to an ancient parchment, its writing partially erased 

 and re-written upon until the process had been repeated several 

 times. 



A. The Preservation of Fossils 



Fossils are the remains, impressions, or petrifactions of living 

 organisms embedded in rock by natural means. This definition 

 has to be slightly qualified or amplified; for the twigs, beads 

 and bits of pottery which are placed in the heavily charged 

 w^aters of Yellowstone Park or Auvergne, France, would not be 

 considered fossils, whereas the mammoths embedded in the ice of 

 Siberia would be. 



It is not necessary that the remains be exceedingly old nor 

 "petrified". A smaller percentage than is usually thought are 

 actually turned to stone. An unusual case of such preservation 

 is the Siberian mammoth, some of which are so well preserved 

 in the strata of ice that one authenticated case of its flesh being 

 eaten is on record, and travellers would have one believe that 

 it is of frequent occurrence. The statement is not unbelievable. 

 These extinct elephants existed in such enormous numbers that 

 the ivory of their tusks is an article of commerce. 



In most of the skeletal remains of mammals there has been 

 no replacement of bone by other substances, although the spaces 

 and cavities have been filled with soluble carbonates or silicates. 

 Microscopic sections show the minutest canaliculi. Similarly 

 the shells of mollusks, and other hard structures, are often simply 

 enclosed by the rock. 



Impressions are also fossils. Fossil leaves and plants are 

 usually formed by the darkening of the rock by the contained 

 carbon. The imprint of the outline and veins was formed when 

 a thin film of mud was laid on the leaf, and eventually pressed 

 into stone. Jelly-fish, although about ninety-eight per cent 



