GROUNDS AND CONSEQUENCES OF EVOLUTION. 339 



metamorphic action has caused the disappearance of all 

 species of fossils. Further, the fossiliferous rocks them- 

 selves have been but very partially explored. No region 

 or locality has been exhausted, while the greater part of 

 the earth remains totally uninvestigated. As a conse- 

 quence of these imperfections in the data, it should be 

 anticipated that the record would present many impassable 

 gaps and apparent anomalies. 



In spite of all this, palaeontology has been able to es- 

 tablish the following principles: 



1. There has been a gradual improvement in the struct- 

 ural rank of the leading types of animals as the history 

 advanced from age to age. 



2. The earlier condition of each animal type was a 

 comprehensive one, in which certain characteristics of two 

 or more later families or orders were united in one 

 species. 



3. The tendency of change has been toward the reso- 

 lution of comprehensive types, so that the characteristics 

 of each separate familv or order should finally be embodied 

 in separate species. 



4. While this process of resolution of comprehensive 

 types has been in progress, still further differentiations 

 and specializations, both in the comprehensive and the 

 resolved forms, have taken place. 



5. The progress of discovery has gone so far that we 

 have established not only a steady progression upward 

 in the animal series at large, but also in several separate 

 ramifications of the series. 



6. Thus we trace tolerably continuous lines of succes- 

 sion, (a) From typical land saurians upward through 

 Pterosaurs, Archceopteryx and Hesperornis to Carinate 



