156 W. K. BROOKS. 



The morphologist 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 feels absolutely certain 

 of the past existence of a stem-form, from which the classes of 

 echinoderms have inherited the fundamental plan of their structure, 

 and he affirms with equal confidence that the structural changes 

 which have separated this ancient type from the classes which we 

 know were 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 under- 

 gone is an index to the length of time which the change has re- 

 quired, and that the period which is covered by the fossiliferous 

 rocks is only an inconsiderable part of that which has been con- 

 sumed in the evolution of the echinoderms. 



The morphologist 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 further back in the 

 past, all the echinoderms were represented by a minute pelagic 

 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 more 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. 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 

 lingula 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. 



