FOSSIL ANIMALS 277 



about what kinds of living creatures, both plants and ani- 

 mals lived in those and supceeding ages. It knows this by 

 virtue of the preservation of parts of some of these animals 

 and plants as fossils. 



Animal fossils are the actual remains of bones or shells 

 or other (usually hard) parts of the body preserved intact in 

 soil or rock; or else, and more commonly, are parts of ani- 

 mals which have been turned into stone by slow replacement 

 of these parts by rock particles; or else, finally, are parts of 

 which stony casts have been made. Examples of these three 

 kinds of fossil are (i) extinct insects preserved in amber, 

 teeth of ancient sharks, tusks of mammoths, shells of various 

 molluscs; (2) petrified bones, corals, crinoids shells, etc.; 

 (3) casts of insect wings, etc. 



Huxley said that " fossils are only animals and plants which 

 have been dead rather longer than those which died yester- 

 day." This "rather longer" may mean anywhere from a 

 few thousand to several milHon years. Geologists estimate 

 the age of the habitable earth — 'the time, that is, since life 

 could have existed on the surface — as being from fifteen mil- 

 lion to seventy miUion years. This enormous time is divided 

 into certain periods of various lengths each period being 

 characterized by a certain set of geologic, geographic and 

 life conditions, and these conditions determining in some 

 measure the kinds of plants and animals living during the 

 period. The geologic history of the earth, which is a very 

 broken and partial one, is read by the geologists from the 

 kiiid and succession of rocks and fossils which form the 

 outer crust. Only in certain rocks, those that have been 

 slowly deposited in water as small (usually soil) particles 

 and have become compacted and hardened into layers or 

 strata, one above the other, do fossils occur. Hence only 

 water-inhabiting animal kinds, or those land kinds whose 

 dead bodies might get into lakes or oceans, are represented 

 by fossil remains. Also, sedimentary or stratified rocks 



