SALPA IN RELATION TO EVOLUTION OF LIFE. 157 



As measured by the time-staudards of the morphologist, lingula 

 itself is modern, for its life-history still holds, locked up within it, 

 the record of a structure and of a habit of life which were lost in 

 the unknown past at the time of the lower cambrian, and it tells 

 us, vaguely but unmistakably, of a life at the surface of the primi- 

 tive ocean at a time when the brachiopod stem was represented by 

 minute and simple pelagic animals. 



Broadly stated, the history of each great line of metazoa has been 

 like that of the echinoderms or brachiopods, for while the brachio- 

 pods are certainly much more closely related to the polyzoa or the 

 gephyreans than to the echinoderms", and while these latter are 

 nearer to the chordata than to the brachiopods, yet each great line 

 stands sharply by itself. 



The oldest pteropod or lamellibranch or crustacean or echinoderm 

 or vertebrate which we know from fossils exhibits its own type of 

 structure with perfect distinctness, and later influences have done 

 no more than to expand and diversify the type, while anatomy fails 

 to guide us back to the point where these various lines met each 

 other in a common source, although it forces us to believe that this 

 common source once had an individual existence. 



Embryology teaches that each line once had its own pelagic 

 representatives, and that the early stages in the evolution of each 

 type have passed away and left no record. 



The palseontological side of the subject has recently been ably 

 summed up by Walcott in an interesting memoir on the oldest 

 fauna which is known to us from fossils (The Fauna of the Lower 

 Cambrian or Oleuellus Zone, by Charles Doolittle Walcott, U. S. 

 Geological Survey, 10th Annual Report, Washington, 1890). 



The fossils of the lower cambrian are not absolutely the oldest 

 known, but it is the oldest fauna which is represented with sufficient 

 completeness for a general view, and is, therefore, interesting to 

 biologists. 



Walcott says that no plants are known in the rocks of the lower 

 cambrian, and that he has satisfied himself, after a study of all the 

 reputed species of algse, that they are not plants, but the trails of 

 worms or molluscs. 



The number of species is small, but their diversity is most note- 

 worthy and remarkable. 



