362 THE ORIGIN OF THE OLDEST FOSSILS. 



The zoolog-ist iiiilicsitatiiigly projects his imagination, held in check 

 only by the laws of scientific thought, into the dark period before the 

 times of the oldest fossils, and he feels absolutely certain of the past 

 existence of a stem from Avhich the classes of echinoderms have 

 inherited the fundamental ]»lan of their structure. He affirms with 

 equal confidence that the structural changes which have separated this 

 ancient type from the classes which we know from fossils are very much 

 more profound and extensive than all the changes which each class has 

 undergone from the earliest paleozoic times to the present day. He is 

 also disi>()sed to assume, but, as I shall show, with much less reason 

 that the amount of 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 inconsid- 

 erable part of that which has been consumed in the evolution of the 

 echinoderms. 



The zoologist does not check the flight of his scientific nnagination 

 here, however, for he trusts iinjdicitly to the embryological evidence 

 which teaches him that still farther back in the i)ast all echinoderms 

 were represented by a minute floating animal which was not an echino- 

 derm at all in any sense except the ancestral one, although it was dis- 

 tinguished by features which natural selection 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 inheritance which has been handed down from genera- 

 tion to generation 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 Chesapeake Bay 

 under conditions which have not eflected 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 efl'ort to grasp its 

 immeasurable antiquity; by the thought that while it has passed 

 through all the chances and changes of geological history, the struc. 

 ture which fltted it for life on the earliest paleozoic bottom is still 

 adapted for a life on the sands of the modern sea floor 1 



The everlasting hills are the type of venerable anticpiity; 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 i)resent 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 tbe 

 record, repeated in the development of each individual, of a structure 

 and a habit of life which were lost in the unknown ]>ast at the time of 

 the Lower Cambrian, and it tells ns vaguely but unmistakably of life 

 at the surface of the i)rimitive ocean at a time when it was represented 

 by minute and simple floating ancestors. 



