W. K. BROOKS ON THE GENUS SALPA. 165 



tion to generation through all the ages from the prehistoric times of 

 zoology. 



Other groups tell the same story with equal clearness. Who can 

 look at a living lingula without being overwhelmed by the effort to grasp 

 its immeasurable antiquity, and by the thought that, while it has passed 

 through all the chances and changes of geological history, the structure 

 which fitted it for life on the earliest palaeozoic bottom is still adapted 

 for a life in the sands of the modern sea-floor ? 



The everlasting hills are the type of venerable antiquity; but lin- 

 gula has seen the continents grow up, and has maintained its integrity 

 unmoved by the convulsions of nature which have given to the crust of 

 the earth its present form. 



As measured by the time-standards 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 primitive 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 brachiopods 

 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 repre- 

 sentatives, and that the early stages in the evolution of each type have 

 passed away and left no record. 



The palaeontological 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 

 Olenellus Zone, by Charles Doolittle Walcott, U. S. Geological Survey, 

 10th Annual Report, Washington, 1890). 



