THE ORIGIN OF THE OLDEST FOSSILS, ETC. 273 



The zoologist does not check the flight of his scientific imagi- 

 nation here, however, for he trusts impHcitly to the embryological 

 evidence which teaches him that, still farther back in the past, all 

 echinoderms were represented by a minute floating animal which 

 was not an echinoderm at all in any sense, except the ancestral 

 one, although it was distinguished by features which natural selec- 

 tion has converted, under the influence of modern conditions, into 

 the structure of echinoderms. He finds in the embryology of 

 modern echinoderms phenomena which can bear no interpretation 

 but this, and he unhesitatingly assumes that they are an inheri- 

 tance which has been handed down from generation to generation 

 through all the ages from the prehistoric times of zoology. 



Other groups tell the same story with equal clearness. Lingula 

 is still living in the sand bars and mud flats of the Chesapeake 

 Bay, under conditions which have not effected any essential change 

 in its structure since the time of the lower Cambrian. Who can 

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

 grasp its immeasurable antiquity ; 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 on the sands of the modern sea floor ? 



The everlasting hills are the type of venerable antiquity ; but 

 lingula has seen the continents grow up, and has maintained its 

 integrity unmoved by the convulsions which have given the crust 

 of the earth its present form. As measured by the time-standards 

 of the zoologist, lingula itself is modern, for its life-history still 

 holds locked up in its embryology the record, repeated in the 

 development of each individual, of a structure and 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 life at 

 the surface of the primitive ocean, at a time when it was repre- 

 sented by minute and simple floating ancestors. 



Broadly stated, the history of each great line has been like that 

 of the echinoderms and brachiopods. The oldest pteropod or 

 lamellibranch or echinoderm or crustacean 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 ex- 

 pand and diversify the type, while anatomy fails to guide us back 



