CHAPTER LXXI 



PAST DISTRIBUTION OF ANIMALS 



PALEOZOOLOGY 



As suggested in the previous chapter (Sec. 570) the present distribu- 

 tion of animals on the earth is determined in part by their past distribu- 

 tion. Those of the present are the descendants of those of the past. 

 Though a few hving types are of relatively recent origin many have 

 existed for many millions of years. The study of the organisms which 

 have lived during the past ages is the field of paleontology, which may be 

 divided into paleobotany and paleozoology, depending upon whether 

 the organisms the remains of which are studied were plant or animal. 



581. Fossils. — Fossils, which means literally objects dug up, are the 

 remains of plants or animals, or records of their presence, preserved in 

 the rocks or in soils. Aside from the fossils commonly found in rocks, 

 they include mammoths, related to present-day elephants, frozen in the 

 soils of northern Siberia; other mammals buried in peat bogs in different 

 parts of the earth; animals found submerged in a lake of asphalt in south- 

 ern California; and insects, scorpions, and spiders, preserved in amber, 

 a fossil resin from the shores of the Baltic Sea. 



582. Stages in Fossilization. — A dead animal is soon eaten by other 

 animals or is destroyed by the processes of decay, even skeletal parts 

 being subject to destruction. If buried in the soil, however, particularly 

 if air cannot reach them, such remains are preserved for a much longer 

 time. Bog waters are antiseptic and in them decay takes place very 

 slowly. Animals are preserved for a long time in volcanic ashes or 

 deposits of fine wind-blown soils. If dead organisms are quickly buried 

 in the mud at the bottom of bodies of water and especially at the bottom 

 of the sea, disintegration is exceedingly slow. Skeletal structures may be 

 preserved for a very long time, and even soft parts may remain long 

 enough to allow mineral matter to replace the organic matter and thus in 

 its arrangement reflect the structure of the organism. Later, as this 

 mud becomes consolidated and forms rock, owing to the pressure of 

 overlying strata and the cementing of the particles together through 

 precipitation of mineral substances from solution, all the organic matter 

 becomes replaced by mineral matter and the fossil is said to have become 

 petrified. In this condition it will last until the rock containing it is 

 broken up by changes in the earth's crust, reduced again to dust by 

 erosion, or metamorphosed (Sec. 585) by the action of heat. Mud 



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