128 INTRODUCTION TO EVOLUTION 



("shells") of arthropods (lobsters, crayfishes, spiders, insects, and their 

 relatives). 



After an animal dies its flesh is destroyed by predatory animals, scaven- 

 gers, insect larvae, bacteria, and so on. These destructive forces also act 

 upon bones and shells, but more slowly. Occasionally such "hard parts" lie 

 in surroundings which protect them from complete destruction, particularly 

 if the animals lived in the water or, in the case of terrestrial animals, if the 

 bones or shells were swept into a body of water by a river at time of flood. 

 The organic matter in bone or shell gradually disintegrates, leaving the 

 structure somewhat porous. Water seeps into the interior of the bone, and 

 minerals dissolved in the water are slowly deposited there. Thus the porosi- 

 ties gradually become filled with deposits of such materials as lime and 

 silica. The portions of the original structure composed of inorganic mate- 

 rials frequently remain substantially as they were in life. In this way even 

 fine details of internal structure may be preserved. 



At times the original material of the bone or shell is dissolved away en- 

 tirely and replaced by other materials. Thus the calcium carbonate compos- 

 ing a shell may in some cases be replaced by silica. The replacing material 

 may preserve the details of the original structure with great fidelity, or, on 

 the other hand, it may preserve only the general form of the original. An 

 example is afforded by the remarkably complete and uninjured inverte- 

 brate fossils now being recovered from limestone rock deposits in the Glass 

 Mountains of Texas. Advantage is taken of the fact that the original mate- 

 rials of shells and exoskeletons have been replaced by silica in the man- 

 ner just described. Pieces of limestone bearing the fossils are immersed in 

 large vats of muriatic acid. The limestone dissolves, in a reaction familiar 

 to every student of high school chemistry, freeing the silicified fossils, 

 intact and undamaged. 



Under exceptionally favorable circumstances replacement of the type 

 under discussion may even result in preservation of some of the internal 

 organs ("soft parts") of an animal. 



Natural preservatives have sometimes helped to save animal materials 

 from destruction. Thus the bones of animals which became mired in the 

 asphalt or tar pits at Rancho La Brea in California are in a fine state of 

 preservation owing to the preservative action of the crude asphaltic oil. In 

 Poland skeletons of the woolly rhinoceros, with some of the flesh and skin 

 preserved, have been found buried in oil-soaked ground. 



Not infrequently the buried body and skeleton of an animal disintegrate 

 entirely. If the surrounding material is sufliciently firm a cavity may re- 

 main having the exact outlines of the structure which disappeared. Such a 



