82 DOCTRINE OF EVOLUTION 



and inorganic agencies which destroy the soft parts, 

 and when a form like a lobster or a crab, possessing a 

 body protected by closely joined shell segments, falls 

 to the bottom of the sea, the chances are that much of 

 the animal's skeleton will be preserved. Thus it is 

 that corals, Crustacea, insects, mollusks, and a few other 

 kinds of lower forms constitute the greater mass of 

 invertebrate palaeontological materials because of their 

 supporting structures of one kind or another. Perhaps 

 the skeletal remains of the vertebrates of the past 

 provide the student of fossils with his best facts, on 

 account of the resistant nature of the bones themselves, 

 and because the backboned animals are relatively mod- 

 ern ; then, too, the rocks in which their remains occur 

 have not been so much altered by geological agencies, 

 or buried so deeply under the strata formed later. 

 Of course only the hardest kinds of shells would remain 

 as such after their burial in materials destined to turn 

 into rock; in the majority of cases, an entombed bone 

 is infiltrated or replaced by various mineral substances 

 so that in time little or nothing of the original thing 

 would remain, though a mold or a cast would persist. 

 But even if an animal of the past possessed hard 

 structures, it must have satisfied certain limited condi- 

 tions to have its remains prove serviceable to students 

 of to-day. A dead mammal must fall upon ground that 

 has just the right consistency to receive it ; if the soil is 

 too soft, its several parts will be separated and scattered 

 as readily as though it had fallen upon hard ground 

 where it would be torn to pieces by carnivorous ani- 

 mals. The dead body must then be covered up by a 

 blanket of silt or sand like that which would be depos- 



