2l8 THE FOUNDATIONS OF ZOOLOGY 



lines distinctly laid down. The Lower Cambrian Crustacea, for 

 example, are as distinct from the Lower Cambrian echinoderms or 

 pteropods or lamellibranchs or brachipods as they are from those of 

 the present day, but zoology gives us evidence that the early steps 

 in the establishment of these great lines were taken under condi- 

 tions which were essentially different from those which have pre- 

 vailed, 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 which 

 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 modifications, 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 their 

 plan of structure is always the same. Paleontology gives us most 

 valuable evidence regarding the course of evolution within the 

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

 to it in vain for light upon the organization of the primitive echino- 

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

 tions on these subjects, and on the relation of the echinoderms 

 to other animals, paleontology 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 echino- 

 derms 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 

 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 



