THE ORIGIN OF THE OLDEST FOSSILS, ETC. 459 



echinoderms to other animals, palaeontology is silent, and throws 

 them back upon us as unsolved riddles. 



The zoologist unhesitatingly 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 which the classes 

 of echinoderms have inherited the fundamental plan 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 palaeozoic times to the present day. 



He is also disposed 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 fossil- 

 iferous 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 

 imagination here, however, for he trusts implicitly to the embryo- 

 logical 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 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 generation 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 effected any 

 essential change in its structure since the time of the lower 

 Cambrian. Who can look at a living lingula without being 



