272 THE ORIGIN OF THE OLDEST FOSSILS, ETC. 



early steps in the establishment of these great lines were taken 

 under conditions which were very different from those which have 

 prevailed, without any essential change, from the time of the oldest 

 fossils to the present day, and that most of the great lines of 

 descent were represented in the remote past by ancestors which, 

 living a different sort of life, differed essentially, in structure as 

 well as in habits, from the representatives of the same types w^hich 

 are known to us as fossils. 



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

 by abundant fossils, very rich in living forms, very diversified in 

 its modification, 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 fossiUferous rocks echinoderms are found, and their plan 

 of structure is always the same. Palaeontology gives us most 

 valuable evidence regarding the course of evolution within the 

 limits of a class as in the crinoids or echinoids ; but we appeal to 

 it in vain for light upon the organisation of the primitive echino- 

 derm, or for connecting links between the classes. To our ques- 

 tions on these subjects, and on the relation of 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 struc- 

 ture. 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 under- 

 gone 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 con- 

 sumed in the evolution of the echinoderms. 



