NATURAL SELECTION, AND THE ANTIQUITY OF LIFE 219 



change which structure has undergone is an index to the length 

 of time which the change has required, and that the period which 

 is covered by the fossiliferous rocks is only an inconsiderable part of 

 that which has been consumed in the evolution of the echinoderms. 



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

 nation here, however, for he trusts implicitly 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 interpreta- 

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

 heritance which has been handed down from generation to gen- 

 eration through all the ages from the prehistoric times of zoology. 



Other groups tell the same story with equal clearness. A 

 lingula is still living in the sand-bars and mud-flats of the Chesa- 

 peake Bay under conditions which have not effected any essen- 

 tial change in its structure since the time of the Lower Cambrian. 

 Who cart 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 earli- 

 est paleozoic 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 em- 

 bryology the record, repeated in the development of each individ- 

 ual, 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 represented by minute and simple floating 

 ancestors. 



