PALEONTOLOGY AS A DISCIPLINE. 49 



or plant to be entombed there is a lucky accident. If all we 

 could learn of the terrestrial life of North America had to be 

 deciphered from the fragments enclosed in the oceanic deposits 

 along its shores, how very imperfect would our knowledge be ! 

 Although the estuarine, swamp, and lake formations, which 

 occur on such a grand scale among the rocks of the earth's 

 crust, have preserved whole chapters in the history of terrestrial 

 life with wonderful fullness and accuracy, they are all too few 

 and too widely separated to form any complete record. Even 

 in a continuous series of marine deposits, representing vast 

 periods of time, there are sure to be gaps of greater or less 

 importance in the record. Changes in the depth of water and 

 the character of the bottom will drive out one set of forms 

 from that locality and bring in another, which has no genetic 

 connection with the former, which may perhaps return with a 

 renewal of the old conditions. Many groups of organisms are 

 incapable of preservation in the fossil state, except under the 

 rarest conditions conditions which occur so seldom and so 

 widely separated in space and time, as to render hopeless any 

 attempt to reconstruct a continuous story from them. 



The very circumstances under which organisms are preserved 

 in the rocks offer another obstacle to the determination of 

 phyletic series. On examining large collections of fossils from 

 several successive horizons, we find that the majority of the 

 species and even of the genera are confined to one or two for- 

 mations, and that each succeeding fauna is recruited partly by 

 migrations from other regions and partly by the rapid expansion 

 of comparatively few adaptive and plastic types, while most of 

 the forms which were especially well fitted for the older con- 

 ditions die out under the new. The collections are, of course, 

 largely made up from the abundant and dominant species of 

 each horizon, which frequently are not the ancestors of those 

 which will be dominant in the succeeding one. The sudden 

 appearance, as it so often seems to be, of a fully differentiated 

 group is sometimes due to that cause, sometimes to a migration 

 from some other region. Even in phyletic series which are 

 well-nigh complete, there is a tendency for each successive 

 genus to undergo similar cycles of specific variation and this 



