146 ZOOLOGY 



contain the same elements. The only decisive criterion 

 is the presence of characteristic fossils. Thus the fossils 

 enable us to recognize the formations, and the relative 

 positions of the. formations in turn give us relative dates 

 for the fossils. What do we mean by characteristic 

 fossils ? Experience shows that throughout all the ages 

 life has been changing. The various geological levels 

 have their representative remains. The length of time 

 a family, genus, or species may last varies greatly ac- 

 cording to the type concerned ; but whenever we can 

 get an assemblage of species, the geological date becomes 

 relatively exact. In a city, the names of a hundred 

 persons present at a meeting would usually define the 

 date within a year, although some of them might have 

 lived there fifty years. The same principle applies to 

 the fossils, though of course we are dealing with very 

 large units of time. 



Relative 3. There are two qualities which make particular 



geologist of groups of fossils especially useful to the geologist. One 

 different [ s t ] ie likelihood of being preserved. Thus marine shells 



groups >f ....... . ,. 



animals living in shallow water are especially important, since 

 the shells are readily fossilized and exist in situations 

 where they are likely to be covered by mud or sand and 

 preserved. Consequently we find such shells in very 

 many formations, and can compare the sets one with 

 another. At the other extreme are butterflies, of which 

 fewer than 25 species are known fossil, and none of these 

 in more than one place. It is impossible, from such 

 scanty remains, to form any exact idea of the changes 

 in butterfly structure from age to age. The other valu- 

 able quality is that of showing relatively rapid modifica- 

 tion. Thus the mammals have changed conspicuously 

 during periods which have witnessed very little change 

 in various types of trees. Other organisms are even 



