CLASSIFICATION AND DISTRIBUTION 385 



type from lower to higher forms. The first organisms, appear- 

 ing in the oldest rocks, were simple forms of low structure, 

 while the highest forms of organisms appeared in the most 

 recent ages. While the progress has not been uniformly con- 

 stant, the general trend has always been upward. The inverte- 

 brates, which contain the lower animals, appeared and cul- 

 minated first, while the vertebrates appeared later. Among 

 the vertebrates the fishes appeared in the earlier rocks, the 

 amphibia came next, reptiles and birds followed, and finally 

 the highest group, the mammals, appeared last, with man at 

 the extreme end of the series. It is true that in this long suc- 

 cession of ages, some forms of organisms have degenerated, 

 becoming simpler and finally disappearing, while others have 

 remained constant for immensely long periods of time without 

 any apparent change. But the general tendency of the whole 

 history has been one of progress from a low form to a higher, 

 from the simple to the complex; and the living world to-day 

 represents the culmination of a long period of progress from the 

 earliest times. This progress, as disclosed by the fossils buried 

 in rocks, is, in a very general way, parallel to the progress of 

 the individual animal as it develops from the egg, through the 

 series of changes which we have learned to call embryology. 

 The parallel between embryology and paleontological history 

 has been one of the striking discoveries of biological study, 

 and has been one of the great factors in the disclosure of the 

 unity of the living world during these long ages. All the facts 

 to-day assure us that there have been uniform laws and forces 

 extending through the whole series of living organisms, from 

 the earliest geological ages to the present, and from PROTOZOA 

 to MAN. 



