164 JOHNS HOPKINS UNIVERSITY MORPHOLOGICAL MONOGRAPHS. 



In the echinoderms we have a well-defined type represented by 

 abundant fossils, very rich in living forms, very diversified in its modifi- 

 cations, and therefore well fitted for use as an illustration. 



This great stem contains many classes and orders, all constructed on 

 the same plan, which is sharply isolated and quite unlike the plan of 

 structure in any other group of animals. All through the series of 

 fossiliferous rocks echinoderms are found, and the plan of structure is 

 always the same. Palaeontology gives us most valuable evidence regard- 

 ing the course of evolution within the limits of a class as in the cri- 

 noids and in the echinoids; but we appeal to it in vain for light upon 

 the organization of the primitive echinoderm, or for connecting links 

 between the classes. To our questions on these subjects and on the rela- 

 tion of the echinoderms to other animals, paleontology is silent, and 

 throws them back upon us as unsolved riddles. 



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 under- 

 gone from the earliest paleozoic 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 fossiliferous rocks is only an inconsid- 

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

 echinoderms. 



The morphologist does not check the flight of his scientific imagina- 

 tion here, however, for he trusts implicitly to the embryological 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 echino- 

 derm 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 echino- 

 derms. 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- 



