PALEONTOLOGY 859 



of the rapid disappearance of a species from the face of the earth 

 is the tragic case of the passenger pigeon. 



Working on this basis, the paleontologist often will use the 

 ''top" of an extended horizon as a fairly precise marker. This 

 method is especially favorable for the micropaleontologist working 

 with well samples, as the drill encountei-s the top first. 



Biologists working exclusively with living organisms or at least 

 living species sometimes are puzzled by the conventions which paleon- 

 tologists have developed. Show a new flower to a botanist and his 

 first effort is to place the specimen in the family to which it belongs. 

 Show an insect to a zoologist and his first thought is to refer it to its 

 proper order. Just as the botanist recognizes families and the zoologist 

 recognizes orders the paleontologist recognizes (or attempts to do 

 so) both of these divisions in his fossils. But the working unit in 

 paleontology is the genus. Undoubtedly, many of the so-called genera 

 of fossils, if we could see the animals restored to life, might actually 

 be families or in some cases might be species. 



The old question of the biologist of "what is a species?" is even 

 more difficult to answer in the ease of fossils. A convention which 

 is fairly well adopted is to consider the different examples of a num- 

 ber of varying fossils as the same species if the varying characters 

 exhibit gradations between the extremes. Thus, sometimes, quite 

 variable fossils are put in the same species. On the other hand, 

 examples which do not appear superficially to differ very greatly 

 may be placed in separate species because certain differing charac- 

 ters do not intergrade. 



Some paleontologists consider, also, that vertical range should be 

 taken into consideration. The time factor is an important one in 

 all branches of geology. No one has ever seen a part of any living 

 species of plant or animal change into a new species. Such changes 

 are common in fossils. No observant person denies the customary 

 assumption that the Kaibab squirrel of the north rim of the Grand 

 Canyon descended from an isolated colony of the more familiar 

 Abert squirrels. This change required many thousands of years, 

 yet in a vertical cliff face of forty or fifty feet, exposing ancient 

 sea bottoms, we may see a record of a quarter of a million years of 

 the history of the sea and its life. 



The paleontologic picture, however, is a badly distorted one, and 

 the imagination must be exercised vigorously to reconstruct the 



