LIFE ON THE EARTH. 81 



SUCCESSIVE SYSTEMS OF MARINE IN VERTEBRAL 

 LIFE. 



The system of life thus constituted in the seas of 

 the most ancient period so far resembles the system 

 now established in modern oceans as to contain the 

 same classes with similar functions and dependen- 

 cies. But there are great differences in the relative 

 proportions of the classes, and of the tribes which 

 are included in them. And these differences are to 

 a certain extent dependent on the elapsed time; 

 classes at first very small have grown very large, 

 others once predominant have been greatly dimi- 

 nished. To make this evident it will be useful to 

 give up the mere enumeration of species in each 

 class, and to adopt as a basis of representation the 

 proportions in which the classes stand to each other 

 in each period. 



This can be easily done by equating the sum of 

 the species in each period to 1000, and the number 

 of species in each class to its proportion of the 

 whole. Choosing for this purpose eight principal 

 classes or assemblages, and tracing their relative 

 proportions in successive periods, we arrive at the 

 result represented in the parallelogram, Fig. 6. 



In this representation of the relative proportions 

 of the several classes in successive geological periods, 

 R. L. G 



