480 THE CHANGING WORLD 



Fossils as Time Markers 



Just as the inclusion of contemporary documents of various sorts 

 within the corner stone of a building, or the carving of a date over 

 the door, indicates the time when the building was erected, so the 

 presence of fossils, found embedded within a particular layer of sedi- 

 mentary rock, serves to fix the approximate time when the sedimenta- 

 tion occurred. 



Since fossils succeed each other over long reaches of time in a cumu- 

 lative series, they aid in establishing the date when a particular layer 

 of the earth's crust was formed, as was first pointed out by William 

 Smith (1769-1839), who succeeded in homologizing certain scattered 

 rock formations in England by means of typical key fossils found in 

 them. Moreover, the kind of stratified rock in which fossils are 

 found in turn helps to determine when the organisms which resulted 

 in fossils lived. Thus, the confirmation works both ways. This is 

 not as much of a vicious circle as it may seem to be, for the progres- 

 siveness, or upward evolution, of organic forms is not taken advantage 

 of in estimating the relative ages of different strata until after the 

 strata themselves have been surely arrayed, by painstaking obser- 

 vation and interpretation, in their unmistakable natural order of 

 occurrence. 



The Testimony of Extinct Types 



A ruined castle on the Rhine, with broken battlements and tumbling 

 towers, is a mute witness to many years employed first in building, 

 followed by a probably extended period of occupation, and by a final 

 period of gradual decay and abandonment. It is quite unlike the 

 flimsy tent of the camper, which is quickly put up at night and taken 

 down in the morning. The castle stands for the lapse of time. The 

 tent does not. The same story of the flight of time is told more 

 emphatically by fossil animals and plants. 



While there have been innumerable individual animals and plants 

 that have lived and died in the past, usually without leaving any trace 

 of their former existence, there are also whole groups of organisms, 

 that is, species, genera, families, orders, and even classes, which have 

 likewise become entirely extinct, and are now known to have existed 

 only because of the occasionally fossilized remains of their representa- 

 tives. To have developed these extensive groups by any process of 

 evolution, and then to allow time enough for the bringing about of 



