INTRODUCTION 7 



men, and that such a land form as Archaeopteryx — the earliest 

 known bird — is represented by only two specimens and a 

 separate feather, all from the lithographic limestone quarries 

 (Upper Jurassic) of Solenhofen, Bavaria. Species of fossil 

 marine shells, on the other hand, are known by thousands of 

 individuals. 



2. That hard parts be present in the organism. 



To insure preservation it is usually necessary that the organ- 

 ism have some protective structure, such as the shell of a clam, 

 the bones of a reptile, or the woody fiber of a tree, which upon 

 the death of the animal or plant resists decay for a much longer 

 time than do the softer portions. Usually the less fibrous 

 plants and the softer parts of animals — epithelium, nerves, 

 muscles, and even cartilage and horn — quickly suffer decay ; 

 and only the skeleton or the external covering, composed of 

 chitin, silica, or of the carbonate or phosphate of calcium, are 

 preservable. Thus multitudes of animals, such as most proto- 

 zoons, jelly-fish, sea anemones, many bryozoa and mollusks, 

 most worms, the tunicates, most parasites, and the embryos 

 of most animals, which lack such hard parts, are exceedingly 

 rare or entirely wanting as fossils. Since the vast majority of 

 the invertebrates living in the sea or fresh water are naked or 

 provided with very fragile hard parts, it is only the bottom-living 

 forms, which, in their heavy, protective hard parts, are usually 

 preserved. Notwithstanding the rapid decay of all soft tissues 

 they are occasionally preserved, as for example, by freezing in 

 the bog-ice of Siberia, by carbonization (page 13), or even by 

 the taking up of lime phosphate on the part of the epidermal 

 and muscular tissues. Animals without hard parts at times 

 also leave a record of themselves, such as trails (like those due 

 to the tentacles of a jelly-fish), as internal molds (as the sand- 

 fillings of the lobed pouches of the jelly-fish, Fig. 45), or as 

 impressions of the entire jelly-fish, of leaves, etc. 



Only rarely are plants protected by calcium carbonate or 

 silica (page 26); usually their preservation is due to direct 



